I think it depends on the Scots Irish influence.
I've known people to say, both, y'all, and ya all.
But, the ya all is rare.
It's mostly the older people, with deep Scots
Irish roots, who say "ya all".
That's been my experience, at least.
So, take it FWIW.
I get it honestly. Scot-Irish on both sides.
And those of any age deeply rooted in Scotch will say about anything. hee hee :-)
...because it used to be "ye all?"