You should be able to contact your admins so they purge your account. Purged accounts have all the content they ever published scrubbed from all databases on Lemmy. I’m not entirely sure how it works if there’s an instance that is defederated from your home instance after you post something and it gets federated to them, I assume it wouldn’t be deleted in that case so it would still be available online there, but certainly a lot harder to track down.