“all y’all” is a Southeastern US colloquialism, mainly “deep” South, but has been carried to other parts of the Nation because we are a free people and can travel when and where we want.
We do not have to change our style of speech to fit the locality’s sensibilities.
That would be micro-fascism.
#befree
As I said, I’m a 5th generation Texan and I’ve never heard it used. Where are you from?