Yeah. And Y'all also used to be plural. Now it can be singular and we use All Y'all to clarify when we need people to know we mean plural. Language is bonkers.
I have never heard y’all used singular, growing up in the American South. Instead, as I understand it:
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Y’all: You all, referring to a group of people (Can potentially be a subset of a larger group, e.g. talking to one couple at the table among a group of friends). “When are y’all having the wedding again?”
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All y’all: shorthand for “all of y’all” Explicitly referring to “all of the members of the group in question”, requiring that at least one member of said group is being addressed by the speaker. The difference is there are no exceptions (apart from exaggeration) “Ain’t a single one of you innocent, all y’all had a hand in this” or “All y’all need to put on your seatbelts, I ain’t going to jail for any of y’all’s comfort”.