I'm going to go ahead and propose that questions of this type should be considered off-topic, if it is posed without further motivation, but can be on-topic if sufficient prior research has been done, and enough (motivating) context has been given. Both are minimum requirements in order for the question to force potential answerers to simply brute-force search the vast literature on a topic. It's hard to provide exact criteria: I think there is somewhat of a continuum of possible questions, which have to judged on a case-by-case basis.
For example, I think a question of the following form could be on-topic:
I discovered that [mathematician X] worked on [topic Y] during [century/decade of choice], and I'm wondering if he/she/it was aware of [result Z], which is relevant to the work. I tried searching some sources (e.g. collected works) but had a hard time coming up with any results, so I'm asking for some help figuring out whether [mathematician X] was aware of [result Z] or not.
However, something like the following should definitely be off-topic:
Playing around with some equations describing [model X] I obtained the following relation [equation Y]. Has this ever been used by anyone?
Furthermore, it has to be made clear what type of result the OP is asking for: It seems obvious that, usually, the question whether anyone ever considered a certain (minor) result is unanswerable. Hence, it needs to be clear whether the OP is asking for a published result, or perhaps something less readily available (the notebooks of Ramanujan come to mind, though they have also been published by now). If something "more special" is requested, it stands to reason that the OP should also show more effort and be more specific in suggestions on where to look.
All in all, I think that questions that are literally of the form
Has [X] ever been considered before?
Should typically be off-topic, though with a lot of guidance on where to look and sufficient prior effort, related questions can be on-topic (as illustrated by my example above).