Hmm, it's an interesting problem. I'm afraid you are right and there's really nothing left but defederation - on the other hand, then it's the same as with stuff like the parsers that could show deleted reddit messages, or things like waybackmachine, which basically do the same, so the core logic of base lemmy source should be as privacy-respecting as possible.

I remember few years ago when I was reading about Signal that there is some way how you can verify that their server is running on the same code as the one published (and audited heavily), so you can be 100% sure that there were no modifications. Wouldn't something like that be a solution? That would prevent servers from modifying the code that deletes data. I don't know how it works, and I couldn't find it when I tried looking for it again, but assuming such a thing is possible, each Lemmy instance could just have a verify widget on their VCS and you could be sure that this instance really does delete your data, since they didn't modify the deletion code.

But this is just a theorycrafting, I wouldn't really have enough experience to create something like that and I can imagine that it's not an easy thing. But if anyone knows more details about the way Signal verification works, assuming I'm just didn't misunderstood something (since it's literally a memory I have of a single sentence from one random article when I was researching best private messages app), I would love to read more about the way it works!

But yeah, outside of that, I'm afraid that the following set of features is mutually exclusive:

• An user is able to delete their data, and it's guaranteed that they are deleted from everywhere.
• If a lemmy instance dies, it's data is not lost.
• There is not a single centralized authority for anything.

Another option would be to create some kind of reputation system, where self-hosted bots could check for servers that still provide posts and comments that should be deleted, and flag offenders. But that's overengineering anyway, and as I've already said - there's still no way how to stop scraper or anyone from simply copying your data when they see it.