I maintain it’s a real word. A proper contraction of you and all in the southern dialect of American English.
Slang is a word like groovy used to describe something that has nothing to do with actual grooves.
Southern–American English nonstandard second-person plural pronoun
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It was never considered "proper" English; however, as cited above, it fills the need of the otherwise-absent second-person plural. It's very common vernacular in the South and some of the West of the United States. I'm not aware of it commonly being used outside the United States.
An argument for the superiority of "y'all" over "you guys," which fills the same grammatical niche in other parts of the US that "y'all" does in the South.
Economical: one word, one syllable.
Nonsexist.
Gracefully takes a possessive. "Is that you guys's new minivan?" vs. "Is that y'all's new Tesla roadster?"
As an Englishman “y’all” isn’t something that is said in British English. Seeing someone write “y’all” immediately identifies the user as an American.
I remember the last time I travelled in the US around 10 years ago, I only heard “y’all” said in the southern states simply as a contraction of “you all”. However, I see the word being used everywhere and replacing other words as well. I’m seeing people using “y’all” instead of ‘all’, ‘you’, ‘ your’, ‘you’re’, ‘everyone’, ‘everybody’… for example I recently saw the sentence “who is y’all favourite YouTuber?”.