I have a question about the workings of a particular processor (to give some context, it's the 6502, a very commonly used processor). Would such a question be on topic here?
I think it's on-topic if it's a challenging question for you, but can't you simply look up the instructions and how they affect the flags? I wrote and ported a lot of math routines back in the day and all that information was actually necessary to do that job properly. Never had any problems finding it in the manufacturer's documentation (and that was long before the internet was common).
In this case, a quick web search will easily locate a reference that answers that question exactly and authoritatively, so I don't think it would be a good candidate, and would probably vote to close if I was into that sort of thing.
Yes, questions related to particular processors are on topic. We get questions here frequently on details of making PICs work, for example.
No, a 6502 is not a "very commonly used" processor. Where did you ever get that ridiculous idea from? It was popular with ancient cave dwellers along with the 6800 and the Z80, but hasn't seen much use since neolithic times.