We've definitely agreed before that (at least some) historical questions are on topic, and I would urge close voters to refrain from closing such questions simply for being historical unless and until policy changes.
I know from my own experience monitoring astro questions that a nonnegligible number of people seem to think a question can only belong in one place, and that if there is a more specific site that caters to the topic, the question should be migrated away from here. However this is simply not the case. Combined with the intrinsic bias of looking at comment votes (they can't be voted down!) and the fact that most people prowling the review queues are doing so intent on closing questions, and I think it's fair to say some upvoted "history is off topic" comments do not in themselves constitute policy.
That said, I have no problem with us deciding to make history off-topic, and I have no problem with this decision being made solely because we now have an HSM site. The questions would hopefully flourish there, and everyone on this site who thinks history is irrelevant can continue happily living in their bubble in which all the right answers are known from on high right from the beginning.1
In this case, though, we would have to consider several points:
- What should we do with old history questions? They're too old to migrate, and we wouldn't want to flood HSM with a bunch of them all at once anyway. Systematically close them all? Leave them around? Lock them as historical?
- Users would have to be informed about the policy. This means a definitive FAQ meta post, a deprecation warning in the history tag, and/or a special close reason.
- What if HSM fails? Do we go back to accepting historical questions? Do we import their physics questions? This isn't just about contingency planning per se; I think an answer to this question will help define why we do not want history here.
1Strong words, I know. But I maintain that anyone who thinks physics is nothing more than learning presently-accepted facts out of a book is deluding themselves. In order to be a physicist, one must learn how to build new theories and experiments. Historical context can, ideally, provide some insight into how to think when you don't already know the answer. Of course, I don't insist that this site cater to every facet of training physicists, otherwise we'd be morally obliged to become a homework help service too.