If the comment history is considered sacrosanct, it could be nice to be able to go back and retroactively change the username for an account, at least.
On reddit this is a right pain, and really fragments discussions. Ending up in a 7 year old discussion with half the comments [deleted] or "I've overwritten all my comments with this bot" is not brilliant. Doubly so when you're trying to find a solution for a technical problem.
As to your second point, reddit is not a public archive, or a court room, or any sort of entity that I would trust or expect to retain, much less make accessible, "public" discussions, technical or not. They have no obligation to give you anything, and the moment it's profitable for them to switch to pay-to-play, you can bet they will. Steve Huffman, the CEO of reddit, killed all the 3rd party apps accessing reddit, no? Not someone I will place my trust in.
[edit- added the below]
I would also add the court of public opinion is a place no one should be tried, and peoples' privacy trumps your desire for information, so it makes perfect sense that comments would be removed or stripped of a byline. Adding a byline redaction feature would be cool imo.
It's been a few years since I read the GDPR, and I'd have to re-read it again with HN's use case in mind, but I'm reasonably (though not completely) sure that "delete every comment you ever made" isn't covered.