I could imagine a scenario where older posts expire from local instance caches (except for possibly a headline?) and if it comes up in search it’s pulled from the original instance again. I could even envision a scenario where pictures, video, and even text posts expire after a certain amount of time from their original instance (or after a certain amount of time with zero interactions). I could also see mods being able to flag content as important, or maybe after a large number of votes it automatically becomes important, and prevents removal from the instance.

All of this would be configurable by the instance owners of course.