You’re right that no federated system can force every server to delete something. But meaningful deletion doesn’t require 100% enforcement.
Lemmy already has admin‑level purge tools that send federated delete requests, and most servers respect them. A user‑level version of that, or a proper “deleted” ActivityPub signal, would give people far more control than the current soft‑delete model.
Even if a few rogue servers ignore it, the majority of the fediverse would still clear the content, which is a huge improvement over “deleted by creator” placeholders.
Federation doesn’t have to mean no deletion — it just means deletion has to be cooperative instead of enforced.