but outside of your own server pretty much nobody will care. Lemmy is federated over multiple jurisdictions, so even with full deletion implemented there’ll almost certainly be instances which will ignore the deletion request - and it will be completely legal for them to do so
Lemmy also seems to federate your matrix_user_id, that is clear personal data. It does not matter how the data gets to the federated server, this is still user data within the scope of the GDPR. It does not matter that that server does not have an agreement with the user, the instance that would ignore a GPDR related deletion request would be in direct violation of the GDPR. Maybe it can do that without consequences, though.
I completely understand that making Lemmy fully GPDR compliant will probably be impossible, however I don't like the approach of "we will not succeed, so we don't make any attempt". Instances should actually delete data when that is requested, or instance hosts can get fined. For now, Lemmy has bigger issues to solve, but eventually they should do at least a best effort attempt to respect user data.