So when subreddits went 'dark' during the protest, and then people deleted their comment history, the messages on the dark subreddits came back when the protest ended and the subreddit became public again. At that point you can just delete the comments again of course, unless you deleted your account, in which case those necromanced comments stay undead forever.

A week later (after my account was deleted and inaccessible) my full comment history was restored.

Reddit's official response is that it was 'an unfortunate bug' and there's nothing they could do.