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.
Comment history shouldn't be considered sacrosanct. This is just a web forum, nothing happening here needs to preserved for the ages. If Reddit and literally every other forum in existence can manage to allow users to delete comments, Hacker News should be able to manage as well.
Yea, I imagine this is hacker news idolizing themselves a bit, i.e. "we are very smart and this is useful discussion." Unfortunately we don't have the GDPR here in the US, with a right to deletion like in the EU, and I don't expect we'll get those features without some privacy laws.
"Right to deletion" doesn't apply to "anything I want deleted", but only to PII within the context of the GDPR. In my reading "everything I ever posted to HN" doesn't really fall under that. If you have some specific comment for a specific reason then that would be different.
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.
I'm pretty sure right to deletion does include account deletion, which I did not see anywhere as an option, but I could be wrong. As you mention, some of this might be possible by messaging the staff members directly. Last I read, there were like 2 moderators for this site who work their ass off.
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.
IIRC it only specifies deletion of private data, not "account deletion". Those are somewhat different things.
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.