Hot take! I think it would be better to keep this consistent (that is, no per-site option), and:
- Increase priority of closed posts which are solved (to greater than open posts).
- Add the option of a timer (ideally, per category and/or per tag) to automatically archive closed posts.
- Within archived posts, prioritize those with solutions — but not so much that they show over non-archived posts.
Disclaimer: why yes, I do want that auto-archive feature for my own site! But I think it makes sense generally. Anything that’s answered but probably irrelevant should be swept away. And if you don’t want that (your solved questions are evergreen), don’t enable the archiver.