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
Exactly how valid is "Y'all"?
grammaticality - When did “y’all” become improper? - English Language & Usage Stack Exchange
Is “y’all” slang or a real word?
How do y’all feel about the usage of the word “y’all”?
What does all y’all mean?
Is it yall or y’all?
Should I say you all or y’all?
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Is this considered a valid contraction for "all of you" even though it doesn't follow the normal contraction rules?
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?"