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.