I think adding a site setting is still welcome, but it’s not on our list at the moment. I’ll mark this topic as pr-welcome though to more clearly signal that we’re open to adding that.
An approach you could try out with the features that exist today is to configure categories to auto-close topics after a certain period of time (something less than 3 years).
Closed topics are weighted lower than open ones in the relevance search (see some recent discussion about that here).