I grew up in the deepest of the deep South. Never did I ever hear "y'all" refer to a single person. I don't know where this trope originated. "Y'all" is definitely plural.
Seconded. In my entire North Carolina + Georgia upbringing, with frequent visits to Tennessee and back to NC, and some visits to South Carolina, Florida, and Alabama...never once have I heard y'all referring to a single person.
It's literally a contraction for "you all". That doesn't make sense for talking to one person.
