In Is there any evidence supporting or contradicting the theory that Harry is an abused child? I notice that several people were voting to close the question because the question could also be asked of other canons and universes. I'm not sure why that would matter. Even though multiple canons share the same trope, answers would still vary, based on the rules and characters of each universe. Is there an actual rule that disallows questions that could be applied to more than one canon?
I believe the close votes were mostly thrown before the question was edited to reference the abused child angle. In its original form, it merely asked "There's a theory that the whole story is in Harry's imagination. Could it be?". In this form, the answer would be valid to multiple canons and universes without any changes. "Yes, it could".
It took a couple of edits until the question arrived at its present, presentable state, and I'm guessing it's the original that people took exception to.
So, to actually answer your question, I don't believe there's a problem with questions that could apply to multiple canons - take for instance the recent Bechdel Test in LotR question, which can and is applied to many canons. It's just the questions whose answers could apply to any canon which are a problem.
I have never seen or heard of any rule like this. Like you said, answers would vary, and it is clearly in-universe. Now I might simplify that particular question to just ask "Is Harry an abused child?" - that sounds like a more clearly focused question (even though it means the exact the same thing).
If this was not allowed, we could close any question that related to the real world in a similar manner (especially ones like "Does X exist in Universe Y", of which I've seen several).
If you ask multiple questions about multiple canons, as long as their answers are not almsot exactly the same, I'd say you're good.
My guess, the issue isn't so much that there's a rule, it's more like trying to preemptively stop users from rep-mining by copying questions from each other and changing a word or two.
Looking solely at the title of the question:
- Is there any evidence supporting or contradicting the theory that Harry is an abused child, imagining it all?
There's little to nothing in the storyline that has to due to with child abuse, so it initially looks like a case of WildMassGuessing (TVTropes), which we could apply to almost anything with a young(er) protagonist:
- Is there any evidence supporting or contradicting the theory that Luke Skywalker is an abused child, imagining it all?
- Is there any evidence supporting or contradicting the theory that Buffy Summers is an abused child, imagining it all?
- Is there any evidence supporting or contradicting the theory that Bella Swan is an abused child, imagining it all?
- Is there any evidence supporting or contradicting the theory that Frodo Baggins is an abused child, imagining it all? (Okay, I'm kind of stretching with this one)
That said, I'm personally okay this question, since it references an essay explaining why this could be the case with Harry Potter. But the ease of inserting any universe into the question probably makes more than a few of us uneasy due to the broken window effect - that is, bad/low quality posts invite more of the same. If we allow this question, why not allow all those others?