Also what about edits? Does HN keep all versions, or does it overwrite the edited comment with the new contents?
And what happens if someone quotes your comment?
Yes. I assume there are like 20 scampers saving everything posted in HN. Including big search companies, intelligence agencies, alternative interface projects and overenthusiastic users.
Going back to the original question:
I don't remember how HN is implemented. IIRC they are still using text files instead of databases. I'll assume they just mark it as deleted, not a hard delete, but I'm a bit paranoiac. Even if it's deleted, I don't think the swipe the hard disk, or all the backups and old machines that have the last year version, or ...
Anyway, if someone makes a huge mistake, they can email the mods and they may try to take a look and perhaps hide or delete it. Anyway, contacting the 20 scrapers may be more difficult.