I didn't see the thread you're talking about and I agree it's really rude to criticise people for the type of language they use, or even to comment on it (unless maybe it's genuinely difficult for most of the intended audience to understand, and the person posting doesn't realise it).
However… sometimes, comments might be made for reasons that are more nuanced than a simple anti-American snobbery or cliquey bullying. Or at least, those may not be intended, even if that's the effect. Spaces where minority language variants are used are often defensively guarded against the influence of more powerful variants, not because the minority variant is intrinsically better but because people perceive its existence as a distinct variant is under threat from dominant cultural forces. Since American English is the dominant variant of English online, that reaction can happen on MN, as one of the few primarily British-English places people chat online — people will make unpleasant comments about perceived dominant-variant users (who here are a minority, so really shouldn't be picked on), and sometimes even more vociferously about minority-variant users who are seen to have picked up features of the dominant variant. It's a misguided urge to defend a less culturally powerful way of speaking, and since language is often felt as a core part of someone's identity, the urge is easily triggered.
The other thing that might sometimes result in criticism of "y'all" is that it's commonly used by a particular subset of people who otherwise use British English, along with "folks", and a few other words and grammatical choices, all with a distinctly American-ish style (I'm aware "folk" and "folks" are used by some British English speakers, but it's slightly different), in a way that signals belonging to a loose social and political grouping of young, very online people with shared beliefs around politics, identity, and so on. This group tends to have some beliefs which clash with those of some MN users, so I think there may be an instinctive rising of hackles because of those associations.
I hope this doesn't come across as a defence of people who criticise others' use of the language that comes naturally to them — that's not my intention. I'm in need of sleep 😅 (which might also explain the rambling sentences that I'm struggling to corral into anything comprehensible).