I believe piefed instances will keep the remote posts within their database for a set amount of days however the post titles will stick around and one can manually fetch the entire posts if they want to see more of them later on.

Like if I see an expired post on piefed.social hosted on Lemmy.world I can see still the title however I will be required to load the rest to view the entirety of it as in the body and comments.

The posts remain fully up on the instances they were posted on IIRC.