When you post stuff on federated instances, it gets copied to the numerous servers that are federating with it. If you delete something, it’ll only get deleted on one server. The other servers might also delete it, but there’s a chance that some servers might still keep the deleted posts. So I’d just say be considerate and think twice before posting, because it can’t be deleted as easily.

https://joinfediverse.wiki/Best_practices#Data_Privacy