Are worldbuilding-style questions which boil down to "is this n-body system I've invented stable" on-topic for this site?
As examples I am talking about things like
Since these questions are in general unanswerable in a definitive fashion, should they be allowed?
I don't think that questions which are "unanswerable in a definitive fashion" are ever good SE questions, no. I think that a question like that can be closed using existing reasons pretty quickly, but it could also be edited, or closed as duplicate to a canonical answer.
These questions linked above are in fact questions about building specific solar systems of their OP's invention, not thought experiments intended to understand astronomical principles, so they're not really about Astronomy and they are really World Building questions.
You could simply ask
Should we start accepting World Building questions?
and to that I'd say we probably shouldn't, because where does it end?
Instead, maybe we can collect a list of answered questions that address stability and offer it as a resource for users wanting to ask about building worlds.
