Just that
Although, by nature of the federation, anything add and then deleted may already be replicated to other instances.
Some of whom could instead show/retain a copy of it.
Basically assume that everything will be available and associated with you forever. Even more so than usual.
Just made me realize that this causes a problem with GDPR. Will that cause issues in the future for the Fediverse?
Maybe? It would depend on the duties imposed on a third party re the GDPR. If your host instance removes your data and a different instance doesn’t, do they have a duty to do so? Do you have to make the request of each instance with a copy?
Is the Wayback machine (https://archive.org/web/) required to remove content it’s archived to be compliant with GDPR?