No, no, no...Y’all is plural. This comes from a GRITS. (Girl Raised in the South-Alabama)
You can say, “Y’all are illiterate or, for emphasis, all y’all are illiterate”. The singular is “you” are illiterate.
The spelling error I hate the most is seperate. My mom taught me that there is a rat in separate.
There's also a rat in Democrat