Hot on the heels of LCS (https://lemmy.world/post/557346), I bring you Lemmy Post Purger (LPP).
Instances can grow fast, and that can be a problem if you have limited disk space. If you don’t mind losing history that your users have not interacted with, LPP can help. It will purge posts along with their comments and media before a certain time period if no comments, likes, or saves exist for users on your instance.
I think it depends on the implementation of this idea, given the nature of federation. It seems like a good place to start.