Are users data protected on the fediverse?

No, not at all. Any data that is federated out assume it's impossible to delete. Sure you can delete stuff from the original server, but will the federated ones obey the request? Or even get the request in the first place?

Nothing you post on social media should have any expectation of privacy.

It's harder to delete things here than on non-federated services because everything you do here gets copied to lots of other servers, which are supposed to delete things when you do, but it's impossible to guarantee that they always will (on purpose or by mistake).

I once deleted a comment here almost immediately after saving it, but then still got multiple upvotes for it. I found out that this was because one big instance hadn't deleted it for whatever reason and its users had no idea that I'd meant to delete it.

Don't think so. There is also nothing stopping an instance admin to patch Lemmy to ignore delete requests.

Well-behaved server software honors delete requests, but there are a bunch of ways for that to fail without anyone doing anything malicious:

• If your instance shuts down, there is no way for you to generate delete requests
• If a server admin has to restore a backup from before your request, the deleted data will be restored
• Immature or experimental software may not work as designed; Lemmy itself has a version number starting with 0
• Archiving services may keep snapshots of pages from fediverse servers; here's your user page on lemmy.world on archive.org
• Fediverse servers often make content available by RSS, and RSS clients may store that content; there's no way for them to receive a signal that it should be deleted

And then there's malicious activity. It wouldn't be hard to run a server that speaks ActivityPub, subscribes to a bunch of stuff, pretends to honor delete requests, and actually keeps everything.

Deletion will always be unreliable on the fediverse as long as it runs on technology that looks anything like current implementations.

Anything you post here is public as well as your activity through the modlog.

Unfortunately I think it must be assumed in this day and age that any content posted online publicly may be stored indefinitely, on any platform. Even if you delete it, there are bots scraping all your comments, and you won't be able to delete from those databases. So I'm not sure how much utility this feature would have today.

Anything you post on the internet is public knowledge forever. End of discussion. Most people won't care at all, in most cases almost nobody or perhaps even literally nobody will ever even see it, but the harder you try to hide it, the more the Streisand Effect will magnify it until eventually everyone knows about it.

Anyone telling you they'll delete your data from the internet without clarifying that it is in fact impossible, is at worst deliberately lying to you usually for their own benefit, and at best making a promise they literally have zero ability to keep.

I would hope that Fediverse services will never lie to you and tell you your data is deleted, because it can't be.

Treat it similarly to how you should treat posting on any social media service.

If you make a post, that is federated to all kinds of other instances. They might process your delete request, or they might just ignore it and keep old posts stored for as long as they want. It's the same with if you make a post on Reddit, someone sends that thread's URL to the Internet Archive, and now there's a permanent record of your account there.

If you post publicly, expect it will be recorded by someone publicly viewing it, and it will not be guaranteed to be removed.

If your instance goes down, then you have no way of deleting your federated posts on your own. So the posts on my lemmynsfw alt will be up forever.

Is it actually possible to request deletion of something on someone else's computer over the internet, and guarantee that its gone?

Basically, the right to be forgotten? Afaik, on Reddit you have scripts to erase all message and rewrite/redact them with nonsense.

I assume something similar may exist for Lemmy, Piefed, etc.

I think not? Which is kind of a drawback to the fediverse. You gotta be careful what you post cause it's probably gonna be somewhere forever.

I could be wrong though

Deleted comments seem to be saved forever so you can undelete them.

The only really important thing is the user account and it should never cave info about ip address or such that is connecting to it.

Yes, all data on your account can be deleted, by deleting your account.