You can appoint a new mod yourself before leaving, or your own account from a different instance. But if you are the only mod when you delete the account then yes. Instance admins can still apoint someone else though.
Sorry I answered a bit out of context. Right to Erasure applies no matter the size, it's the processing of the data records that only applies to companies with 250+ employees.
And Lemmy is GDPR compliant now as if a post/comment is deleted it is removed within 30 days. But it falls down to each instance that federates to process those delete requests. But deleting your account doesn't delate the content you generated not does it claim to do so.
Lemmy definetely should add an option to delete all content when deleting the account, but I think the reason it doesn't is that it would leave big gaps in conversations under posts (e.g. if I were to delete my comments in this thread, your comments would make no sense for anything else reading then). Alternatively they could just unlink it from the account and just leave the post/comment with some placeholder name like deleted.