If instances A B C are online and federated when you post, then A B C have copies of the data

Let's say A shuts down and you move to a new instance D

Only B and C have your old content. D has none of your data. There is no backfill mechanism

If in the future B and C go offline (eventually they will) then the data is gone. Maybe in a archive.org snapshot.

Even in the future if D E F G still are going strong and Lemmy is thriving all the content that only A B C saw is gone. It's a bitrot problem