

“All y’all” is actually a really interesting linguistic object. It draws a grammaticalized contrast between a 2nd person plural exclusive and a 2nd person plural inclusive, which isn’t really a thing in English elsewhere, even back in Old English when it had more complex morphology.
That is to say, “y’all” is a second person plural, and it basically means “you guys”, or “y’inz” if you’re from that part of Pennsylvania. It picks out a specific group of people that you’re talking directly to.
But, what if I’m talking to a crowd, and I want to draw a distinction between some people among them that I’ve already addressed, versus the group in its entirety, which I would now like to address?
This is where “all y’all” comes in. It essentially specifies that everyone is being addressed, and not just a specific subset of that group.
So you’ll get examples like “Y’all (usually with a pointing motion) need to rethink your design, but all y’all need to listen better”.
@fletcher_bosom@lemmy.world likely intended “all y’all” to have a specifically inclusive meaning here for a few different types of groups that could be distinguished among the readers of their comment. Basically, “not just y’all (the linguists in the group who are irked by their prescriptiveness), but all y’all (everyone else as well, including those who aren’t irked), are lucky that they have one foot in the grave”.
“All y’all” was necessary here because if they’d just said “y’all” it could easily have been misinterpreted as just the educated linguists being lucky, and they wanted to emphasize that their comment wasn’t being snide toward educated people, but rather self-deprecating. “All y’all” accomplishes that perfectly (for those who have it in their grammars).
“All y’all” is one of my favorite constructions in (dialectal) English, though it is hilariously ironic, and kinda demonstrative of my general point, to see such a non-standard feature in a comment arguing in favor of prescriptive grammar rules!
(Also, if you think “all y’all” is weird, wait 'til you see singular y’all!)

That’s not how that works. (Normal) commuting costs are never deductible (in the US). Even if your company covers commuting costs for you, that compensation is still fully taxable as income.