I know absolutely nothing about how Lemmy works but there's hopefully some way or will need to be eventually a way to remove old posts from a certain instance. For instance if there is illegal content it would need to be removed. Building a good solution for this that could be done optionally in the defederation process would probably be good and will be necessary at some point anyway.

For instance you defederate from a nazi server — you'd absolutely want the nazi posts gone from your server. And legally you'd need to potentially depending on where the server was hosted — and of course other kinds of illegal content and networks.

I'm not sure what the current state of the tools is though.