Unknown - Lemmy is a bit less "mature" than some of the other Fediverse platforms like Mastodon where you can at least export your data. All we have is "delete my account" for example.

If the instance goes away, that's a good question. Certainly federated instances stop receiving anything from it. Will instances that have users who have subscribed to its communities still see cached posts etc? Probably. Unless there's a "Im going away, purge my stuff from you" msg in the AP that I dunno of (haven't finished reading it), a subscribing instance might just think "well, its unavailable at the moment Ill try later."