I recently created an array containing all timezones and found that a couple of official timezones were missing from Chromium 71.0.3578.98.

You can see that array in this comment of the bug I reported. For convenience, I've included the array below:

var aryIanaTimeZones = [
  'Europe/Andorra',
  'Asia/Dubai',
  'Asia/Kabul',
  'Europe/Tirane',
  'Asia/Yerevan',
  'Antarctica/Casey',
  'Antarctica/Davis',
  'Antarctica/DumontDUrville', // https://bugs.chromium.org/p/chromium/issues/detail?id=928068
  'Antarctica/Mawson',
  'Antarctica/Palmer',
  'Antarctica/Rothera',
  'Antarctica/Syowa',
  'Antarctica/Troll',
  'Antarctica/Vostok',
  'America/Argentina/Buenos_Aires',
  'America/Argentina/Cordoba',
  'America/Argentina/Salta',
  'America/Argentina/Jujuy',
  'America/Argentina/Tucuman',
  'America/Argentina/Catamarca',
  'America/Argentina/La_Rioja',
  'America/Argentina/San_Juan',
  'America/Argentina/Mendoza',
  'America/Argentina/San_Luis',
  'America/Argentina/Rio_Gallegos',
  'America/Argentina/Ushuaia',
  'Pacific/Pago_Pago',
  'Europe/Vienna',
  'Australia/Lord_Howe',
  'Antarctica/Macquarie',
  'Australia/Hobart',
  'Australia/Currie',
  'Australia/Melbourne',
  'Australia/Sydney',
  'Australia/Broken_Hill',
  'Australia/Brisbane',
  'Australia/Lindeman',
  'Australia/Adelaide',
  'Australia/Darwin',
  'Australia/Perth',
  'Australia/Eucla',
  'Asia/Baku',
  'America/Barbados',
  'Asia/Dhaka',
  'Europe/Brussels',
  'Europe/Sofia',
  'Atlantic/Bermuda',
  'Asia/Brunei',
  'America/La_Paz',
  'America/Noronha',
  'America/Belem',
  'America/Fortaleza',
  'America/Recife',
  'America/Araguaina',
  'America/Maceio',
  'America/Bahia',
  'America/Sao_Paulo',
  'America/Campo_Grande',
  'America/Cuiaba',
  'America/Santarem',
  'America/Porto_Velho',
  'America/Boa_Vista',
  'America/Manaus',
  'America/Eirunepe',
  'America/Rio_Branco',
  'America/Nassau',
  'Asia/Thimphu',
  'Europe/Minsk',
  'America/Belize',
  'America/St_Johns',
  'America/Halifax',
  'America/Glace_Bay',
  'America/Moncton',
  'America/Goose_Bay',
  'America/Blanc-Sablon',
  'America/Toronto',
  'America/Nipigon',
  'America/Thunder_Bay',
  'America/Iqaluit',
  'America/Pangnirtung',
  'America/Atikokan',
  'America/Winnipeg',
  'America/Rainy_River',
  'America/Resolute',
  'America/Rankin_Inlet',
  'America/Regina',
  'America/Swift_Current',
  'America/Edmonton',
  'America/Cambridge_Bay',
  'America/Yellowknife',
  'America/Inuvik',
  'America/Creston',
  'America/Dawson_Creek',
  'America/Fort_Nelson',
  'America/Vancouver',
  'America/Whitehorse',
  'America/Dawson',
  'Indian/Cocos',
  'Europe/Zurich',
  'Africa/Abidjan',
  'Pacific/Rarotonga',
  'America/Santiago',
  'America/Punta_Arenas',
  'Pacific/Easter',
  'Asia/Shanghai',
  'Asia/Urumqi',
  'America/Bogota',
  'America/Costa_Rica',
  'America/Havana',
  'Atlantic/Cape_Verde',
  'America/Curacao',
  'Indian/Christmas',
  'Asia/Nicosia',
  'Asia/Famagusta',
  'Europe/Prague',
  'Europe/Berlin',
  'Europe/Copenhagen',
  'America/Santo_Domingo',
  'Africa/Algiers',
  'America/Guayaquil',
  'Pacific/Galapagos',
  'Europe/Tallinn',
  'Africa/Cairo',
  'Africa/El_Aaiun',
  'Europe/Madrid',
  'Africa/Ceuta',
  'Atlantic/Canary',
  'Europe/Helsinki',
  'Pacific/Fiji',
  'Atlantic/Stanley',
  'Pacific/Chuuk',
  'Pacific/Pohnpei',
  'Pacific/Kosrae',
  'Atlantic/Faroe',
  'Europe/Paris',
  'Europe/London',
  'Asia/Tbilisi',
  'America/Cayenne',
  'Africa/Accra',
  'Europe/Gibraltar',
  'America/Godthab',
  'America/Danmarkshavn',
  'America/Scoresbysund',
  'America/Thule',
  'Europe/Athens',
  'Atlantic/South_Georgia',
  'America/Guatemala',
  'Pacific/Guam',
  'Africa/Bissau',
  'America/Guyana',
  'Asia/Hong_Kong',
  'America/Tegucigalpa',
  'America/Port-au-Prince',
  'Europe/Budapest',
  'Asia/Jakarta',
  'Asia/Pontianak',
  'Asia/Makassar',
  'Asia/Jayapura',
  'Europe/Dublin',
  'Asia/Jerusalem',
  'Asia/Kolkata',
  'Indian/Chagos',
  'Asia/Baghdad',
  'Asia/Tehran',
  'Atlantic/Reykjavik',
  'Europe/Rome',
  'America/Jamaica',
  'Asia/Amman',
  'Asia/Tokyo',
  'Africa/Nairobi',
  'Asia/Bishkek',
  'Pacific/Tarawa',
  'Pacific/Enderbury',
  'Pacific/Kiritimati',
  'Asia/Pyongyang',
  'Asia/Seoul',
  'Asia/Almaty',
  'Asia/Qyzylorda',
  'Asia/Qostanay', // https://bugs.chromium.org/p/chromium/issues/detail?id=928068
  'Asia/Aqtobe',
  'Asia/Aqtau',
  'Asia/Atyrau',
  'Asia/Oral',
  'Asia/Beirut',
  'Asia/Colombo',
  'Africa/Monrovia',
  'Europe/Vilnius',
  'Europe/Luxembourg',
  'Europe/Riga',
  'Africa/Tripoli',
  'Africa/Casablanca',
  'Europe/Monaco',
  'Europe/Chisinau',
  'Pacific/Majuro',
  'Pacific/Kwajalein',
  'Asia/Yangon',
  'Asia/Ulaanbaatar',
  'Asia/Hovd',
  'Asia/Choibalsan',
  'Asia/Macau',
  'America/Martinique',
  'Europe/Malta',
  'Indian/Mauritius',
  'Indian/Maldives',
  'America/Mexico_City',
  'America/Cancun',
  'America/Merida',
  'America/Monterrey',
  'America/Matamoros',
  'America/Mazatlan',
  'America/Chihuahua',
  'America/Ojinaga',
  'America/Hermosillo',
  'America/Tijuana',
  'America/Bahia_Banderas',
  'Asia/Kuala_Lumpur',
  'Asia/Kuching',
  'Africa/Maputo',
  'Africa/Windhoek',
  'Pacific/Noumea',
  'Pacific/Norfolk',
  'Africa/Lagos',
  'America/Managua',
  'Europe/Amsterdam',
  'Europe/Oslo',
  'Asia/Kathmandu',
  'Pacific/Nauru',
  'Pacific/Niue',
  'Pacific/Auckland',
  'Pacific/Chatham',
  'America/Panama',
  'America/Lima',
  'Pacific/Tahiti',
  'Pacific/Marquesas',
  'Pacific/Gambier',
  'Pacific/Port_Moresby',
  'Pacific/Bougainville',
  'Asia/Manila',
  'Asia/Karachi',
  'Europe/Warsaw',
  'America/Miquelon',
  'Pacific/Pitcairn',
  'America/Puerto_Rico',
  'Asia/Gaza',
  'Asia/Hebron',
  'Europe/Lisbon',
  'Atlantic/Madeira',
  'Atlantic/Azores',
  'Pacific/Palau',
  'America/Asuncion',
  'Asia/Qatar',
  'Indian/Reunion',
  'Europe/Bucharest',
  'Europe/Belgrade',
  'Europe/Kaliningrad',
  'Europe/Moscow',
  'Europe/Simferopol',
  'Europe/Kirov',
  'Europe/Astrakhan',
  'Europe/Volgograd',
  'Europe/Saratov',
  'Europe/Ulyanovsk',
  'Europe/Samara',
  'Asia/Yekaterinburg',
  'Asia/Omsk',
  'Asia/Novosibirsk',
  'Asia/Barnaul',
  'Asia/Tomsk',
  'Asia/Novokuznetsk',
  'Asia/Krasnoyarsk',
  'Asia/Irkutsk',
  'Asia/Chita',
  'Asia/Yakutsk',
  'Asia/Khandyga',
  'Asia/Vladivostok',
  'Asia/Ust-Nera',
  'Asia/Magadan',
  'Asia/Sakhalin',
  'Asia/Srednekolymsk',
  'Asia/Kamchatka',
  'Asia/Anadyr',
  'Asia/Riyadh',
  'Pacific/Guadalcanal',
  'Indian/Mahe',
  'Africa/Khartoum',
  'Europe/Stockholm',
  'Asia/Singapore',
  'America/Paramaribo',
  'Africa/Juba',
  'Africa/Sao_Tome',
  'America/El_Salvador',
  'Asia/Damascus',
  'America/Grand_Turk',
  'Africa/Ndjamena',
  'Indian/Kerguelen',
  'Asia/Bangkok',
  'Asia/Dushanbe',
  'Pacific/Fakaofo',
  'Asia/Dili',
  'Asia/Ashgabat',
  'Africa/Tunis',
  'Pacific/Tongatapu',
  'Europe/Istanbul',
  'America/Port_of_Spain',
  'Pacific/Funafuti',
  'Asia/Taipei',
  'Europe/Kiev',
  'Europe/Uzhgorod',
  'Europe/Zaporozhye',
  'Pacific/Wake',
  'America/New_York',
  'America/Detroit',
  'America/Kentucky/Louisville',
  'America/Kentucky/Monticello',
  'America/Indiana/Indianapolis',
  'America/Indiana/Vincennes',
  'America/Indiana/Winamac',
  'America/Indiana/Marengo',
  'America/Indiana/Petersburg',
  'America/Indiana/Vevay',
  'America/Chicago',
  'America/Indiana/Tell_City',
  'America/Indiana/Knox',
  'America/Menominee',
  'America/North_Dakota/Center',
  'America/North_Dakota/New_Salem',
  'America/North_Dakota/Beulah',
  'America/Denver',
  'America/Boise',
  'America/Phoenix',
  'America/Los_Angeles',
  'America/Anchorage',
  'America/Juneau',
  'America/Sitka',
  'America/Metlakatla',
  'America/Yakutat',
  'America/Nome',
  'America/Adak',
  'Pacific/Honolulu',
  'America/Montevideo',
  'Asia/Samarkand',
  'Asia/Tashkent',
  'America/Caracas',
  'Asia/Ho_Chi_Minh',
  'Pacific/Efate',
  'Pacific/Wallis',
  'Pacific/Apia',
  'Africa/Johannesburg'
];

let date = new Date;
aryIanaTimeZones.forEach((timeZone) =>
{
  let strTime = date.toLocaleString("en-US", {timeZone: `${timeZone}`});
  console.log(timeZone, strTime);
});

If you're like me, in wondering why there are so many timezones, check out this question.

UPDATE 2022-06-08:

If you only need a full list of time zones that your environment supports, you can now acquire that array using Intl.supportedValuesOf(), like this:

let ary = Intl.supportedValuesOf('timeZone');

Whether that list is complete or not, will depend on your environment.

In the snippet below, I replace that array (from the first snippet) with Intl.supportedValuesOf('timeZone'):

var aryIanaTimeZones = Intl.supportedValuesOf('timeZone');

let date = new Date;
aryIanaTimeZones.forEach((timeZone) =>
{
  let strTime = date.toLocaleString("en-US",{timeZone: `${timeZone}`});
  console.log(timeZone, strTime);
});

Here's a comparison of the custom array versus Intl.supportedValuesOf('timeZone'):

let ary1 = [
      'Europe/Andorra',
      'Asia/Dubai',
      'Asia/Kabul',
      'Europe/Tirane',
      'Asia/Yerevan',
      'Antarctica/Casey',
      'Antarctica/Davis',
      'Antarctica/DumontDUrville', // https://bugs.chromium.org/p/chromium/issues/detail?id=928068
      'Antarctica/Mawson',
      'Antarctica/Palmer',
      'Antarctica/Rothera',
      'Antarctica/Syowa',
      'Antarctica/Troll',
      'Antarctica/Vostok',
      'America/Argentina/Buenos_Aires',
      'America/Argentina/Cordoba',
      'America/Argentina/Salta',
      'America/Argentina/Jujuy',
      'America/Argentina/Tucuman',
      'America/Argentina/Catamarca',
      'America/Argentina/La_Rioja',
      'America/Argentina/San_Juan',
      'America/Argentina/Mendoza',
      'America/Argentina/San_Luis',
      'America/Argentina/Rio_Gallegos',
      'America/Argentina/Ushuaia',
      'Pacific/Pago_Pago',
      'Europe/Vienna',
      'Australia/Lord_Howe',
      'Antarctica/Macquarie',
      'Australia/Hobart',
      'Australia/Currie',
      'Australia/Melbourne',
      'Australia/Sydney',
      'Australia/Broken_Hill',
      'Australia/Brisbane',
      'Australia/Lindeman',
      'Australia/Adelaide',
      'Australia/Darwin',
      'Australia/Perth',
      'Australia/Eucla',
      'Asia/Baku',
      'America/Barbados',
      'Asia/Dhaka',
      'Europe/Brussels',
      'Europe/Sofia',
      'Atlantic/Bermuda',
      'Asia/Brunei',
      'America/La_Paz',
      'America/Noronha',
      'America/Belem',
      'America/Fortaleza',
      'America/Recife',
      'America/Araguaina',
      'America/Maceio',
      'America/Bahia',
      'America/Sao_Paulo',
      'America/Campo_Grande',
      'America/Cuiaba',
      'America/Santarem',
      'America/Porto_Velho',
      'America/Boa_Vista',
      'America/Manaus',
      'America/Eirunepe',
      'America/Rio_Branco',
      'America/Nassau',
      'Asia/Thimphu',
      'Europe/Minsk',
      'America/Belize',
      'America/St_Johns',
      'America/Halifax',
      'America/Glace_Bay',
      'America/Moncton',
      'America/Goose_Bay',
      'America/Blanc-Sablon',
      'America/Toronto',
      'America/Nipigon',
      'America/Thunder_Bay',
      'America/Iqaluit',
      'America/Pangnirtung',
      'America/Atikokan',
      'America/Winnipeg',
      'America/Rainy_River',
      'America/Resolute',
      'America/Rankin_Inlet',
      'America/Regina',
      'America/Swift_Current',
      'America/Edmonton',
      'America/Cambridge_Bay',
      'America/Yellowknife',
      'America/Inuvik',
      'America/Creston',
      'America/Dawson_Creek',
      'America/Fort_Nelson',
      'America/Vancouver',
      'America/Whitehorse',
      'America/Dawson',
      'Indian/Cocos',
      'Europe/Zurich',
      'Africa/Abidjan',
      'Pacific/Rarotonga',
      'America/Santiago',
      'America/Punta_Arenas',
      'Pacific/Easter',
      'Asia/Shanghai',
      'Asia/Urumqi',
      'America/Bogota',
      'America/Costa_Rica',
      'America/Havana',
      'Atlantic/Cape_Verde',
      'America/Curacao',
      'Indian/Christmas',
      'Asia/Nicosia',
      'Asia/Famagusta',
      'Europe/Prague',
      'Europe/Berlin',
      'Europe/Copenhagen',
      'America/Santo_Domingo',
      'Africa/Algiers',
      'America/Guayaquil',
      'Pacific/Galapagos',
      'Europe/Tallinn',
      'Africa/Cairo',
      'Africa/El_Aaiun',
      'Europe/Madrid',
      'Africa/Ceuta',
      'Atlantic/Canary',
      'Europe/Helsinki',
      'Pacific/Fiji',
      'Atlantic/Stanley',
      'Pacific/Chuuk',
      'Pacific/Pohnpei',
      'Pacific/Kosrae',
      'Atlantic/Faroe',
      'Europe/Paris',
      'Europe/London',
      'Asia/Tbilisi',
      'America/Cayenne',
      'Africa/Accra',
      'Europe/Gibraltar',
      'America/Godthab',
      'America/Danmarkshavn',
      'America/Scoresbysund',
      'America/Thule',
      'Europe/Athens',
      'Atlantic/South_Georgia',
      'America/Guatemala',
      'Pacific/Guam',
      'Africa/Bissau',
      'America/Guyana',
      'Asia/Hong_Kong',
      'America/Tegucigalpa',
      'America/Port-au-Prince',
      'Europe/Budapest',
      'Asia/Jakarta',
      'Asia/Pontianak',
      'Asia/Makassar',
      'Asia/Jayapura',
      'Europe/Dublin',
      'Asia/Jerusalem',
      'Asia/Kolkata',
      'Indian/Chagos',
      'Asia/Baghdad',
      'Asia/Tehran',
      'Atlantic/Reykjavik',
      'Europe/Rome',
      'America/Jamaica',
      'Asia/Amman',
      'Asia/Tokyo',
      'Africa/Nairobi',
      'Asia/Bishkek',
      'Pacific/Tarawa',
      'Pacific/Enderbury',
      'Pacific/Kiritimati',
      'Asia/Pyongyang',
      'Asia/Seoul',
      'Asia/Almaty',
      'Asia/Qyzylorda',
      'Asia/Qostanay', // https://bugs.chromium.org/p/chromium/issues/detail?id=928068
      'Asia/Aqtobe',
      'Asia/Aqtau',
      'Asia/Atyrau',
      'Asia/Oral',
      'Asia/Beirut',
      'Asia/Colombo',
      'Africa/Monrovia',
      'Europe/Vilnius',
      'Europe/Luxembourg',
      'Europe/Riga',
      'Africa/Tripoli',
      'Africa/Casablanca',
      'Europe/Monaco',
      'Europe/Chisinau',
      'Pacific/Majuro',
      'Pacific/Kwajalein',
      'Asia/Yangon',
      'Asia/Ulaanbaatar',
      'Asia/Hovd',
      'Asia/Choibalsan',
      'Asia/Macau',
      'America/Martinique',
      'Europe/Malta',
      'Indian/Mauritius',
      'Indian/Maldives',
      'America/Mexico_City',
      'America/Cancun',
      'America/Merida',
      'America/Monterrey',
      'America/Matamoros',
      'America/Mazatlan',
      'America/Chihuahua',
      'America/Ojinaga',
      'America/Hermosillo',
      'America/Tijuana',
      'America/Bahia_Banderas',
      'Asia/Kuala_Lumpur',
      'Asia/Kuching',
      'Africa/Maputo',
      'Africa/Windhoek',
      'Pacific/Noumea',
      'Pacific/Norfolk',
      'Africa/Lagos',
      'America/Managua',
      'Europe/Amsterdam',
      'Europe/Oslo',
      'Asia/Kathmandu',
      'Pacific/Nauru',
      'Pacific/Niue',
      'Pacific/Auckland',
      'Pacific/Chatham',
      'America/Panama',
      'America/Lima',
      'Pacific/Tahiti',
      'Pacific/Marquesas',
      'Pacific/Gambier',
      'Pacific/Port_Moresby',
      'Pacific/Bougainville',
      'Asia/Manila',
      'Asia/Karachi',
      'Europe/Warsaw',
      'America/Miquelon',
      'Pacific/Pitcairn',
      'America/Puerto_Rico',
      'Asia/Gaza',
      'Asia/Hebron',
      'Europe/Lisbon',
      'Atlantic/Madeira',
      'Atlantic/Azores',
      'Pacific/Palau',
      'America/Asuncion',
      'Asia/Qatar',
      'Indian/Reunion',
      'Europe/Bucharest',
      'Europe/Belgrade',
      'Europe/Kaliningrad',
      'Europe/Moscow',
      'Europe/Simferopol',
      'Europe/Kirov',
      'Europe/Astrakhan',
      'Europe/Volgograd',
      'Europe/Saratov',
      'Europe/Ulyanovsk',
      'Europe/Samara',
      'Asia/Yekaterinburg',
      'Asia/Omsk',
      'Asia/Novosibirsk',
      'Asia/Barnaul',
      'Asia/Tomsk',
      'Asia/Novokuznetsk',
      'Asia/Krasnoyarsk',
      'Asia/Irkutsk',
      'Asia/Chita',
      'Asia/Yakutsk',
      'Asia/Khandyga',
      'Asia/Vladivostok',
      'Asia/Ust-Nera',
      'Asia/Magadan',
      'Asia/Sakhalin',
      'Asia/Srednekolymsk',
      'Asia/Kamchatka',
      'Asia/Anadyr',
      'Asia/Riyadh',
      'Pacific/Guadalcanal',
      'Indian/Mahe',
      'Africa/Khartoum',
      'Europe/Stockholm',
      'Asia/Singapore',
      'America/Paramaribo',
      'Africa/Juba',
      'Africa/Sao_Tome',
      'America/El_Salvador',
      'Asia/Damascus',
      'America/Grand_Turk',
      'Africa/Ndjamena',
      'Indian/Kerguelen',
      'Asia/Bangkok',
      'Asia/Dushanbe',
      'Pacific/Fakaofo',
      'Asia/Dili',
      'Asia/Ashgabat',
      'Africa/Tunis',
      'Pacific/Tongatapu',
      'Europe/Istanbul',
      'America/Port_of_Spain',
      'Pacific/Funafuti',
      'Asia/Taipei',
      'Europe/Kiev',
      'Europe/Uzhgorod',
      'Europe/Zaporozhye',
      'Pacific/Wake',
      'America/New_York',
      'America/Detroit',
      'America/Kentucky/Louisville',
      'America/Kentucky/Monticello',
      'America/Indiana/Indianapolis',
      'America/Indiana/Vincennes',
      'America/Indiana/Winamac',
      'America/Indiana/Marengo',
      'America/Indiana/Petersburg',
      'America/Indiana/Vevay',
      'America/Chicago',
      'America/Indiana/Tell_City',
      'America/Indiana/Knox',
      'America/Menominee',
      'America/North_Dakota/Center',
      'America/North_Dakota/New_Salem',
      'America/North_Dakota/Beulah',
      'America/Denver',
      'America/Boise',
      'America/Phoenix',
      'America/Los_Angeles',
      'America/Anchorage',
      'America/Juneau',
      'America/Sitka',
      'America/Metlakatla',
      'America/Yakutat',
      'America/Nome',
      'America/Adak',
      'Pacific/Honolulu',
      'America/Montevideo',
      'Asia/Samarkand',
      'Asia/Tashkent',
      'America/Caracas',
      'Asia/Ho_Chi_Minh',
      'Pacific/Efate',
      'Pacific/Wallis',
      'Pacific/Apia',
      'Africa/Johannesburg'];

let ary2 = Intl.supportedValuesOf('timeZone');

let s1 = new Set(ary1);
let s2 = new Set(ary2);

let aryNotInSupportedValuesOf = ary1.filter((timeZone)=>
{
  if (!s2.has(timeZone))
  {
    return timeZone;
  }
});

let aryNotInCustomArray = ary2.filter((timeZone)=>
{
  if (!s1.has(timeZone))
  {
    return timeZone;
  }
});

if (aryNotInSupportedValuesOf.length > 0)
{
  console.log (`These values, that are included in the custom array, are not included in Intl.supportedValuesOf('timeZone'):` ,aryNotInSupportedValuesOf);
}
else
{
  console.log(`All values in the custom Array are included in Intl.supportedValuesOf('timeZone')`);
}

if (aryNotInCustomArray.length > 0)
{
  console.log (`These values, provided by Intl.supportedValuesOf('timeZone'), are not included in the custom array:`,aryNotInCustomArray);
}
else
{
  console.log(`All values in Intl.supportedValuesOf('timeZone') are inclued in the custom array.`);
}

Running the comparison above produces different results in each environment/browser I try. The latest Firefox reveals that numerous time zones have been added to the spec since I originally created that static list. However, other environments (I try) are still missing official time zones that were indeed included in that old list.

As of Jul 5th 2022, despite what's reflected on on caniuse or MDN, I see Intl.supportedValuesOf('timeZone') working in the latest:

• Chromium
• Firefox
• Node (v.18.4.0)

If Intl.supportedValuesOf('timeZone') isn't working in the snippets above, for a browser you're targeting, feel free to include a URL to that issue's bug-tracker in the comments below.