ahh, yeah ok - that makes sense. 

Had a quick look at nostr-rs-relay and it looks like all you'd really need is to add a new column to the database to store the last accessed time of the stored event. 

Then you could easily have a background task that ran to clean up. The methods that touch the events would just need to update that field every time the event is returned in a REQed. You'd have to spend a bit of time to make sure you optimized how to do that write... but feels doable to me.