This is a variation of this feature suggestion. It is much more difficult to implement, but is much more refined, as it allows the deletion of specific parts of a comment thread.
Many - but not all - conversations in comments lose their value after the conversation has been concluded - fixed typos, requests for clarification etc. Conversely, there are some conversations that are worth preserving because they add value to the question or clarify a common misconception.
Robert suggests that moderators should as a rule weed out outdated comment threads. While that would probably work most of the time, I fear that judging what can be deleted and what shouldn't often requires domain knowledge, and experience with the issue at hand.
Therefore, cleaning up comments should be in the hands of the people best suited to decide whether a conversation has lasting value: Those who are having it.
To do this, I would like to suggest the following functionality.
Add a "recycling" symbol underneath each comment thread that exceeds two or three comments.
Upon clicking the symbol, the user selects the start and the end point of the conversation they want to see deleted. The selected conversation is highlighted (in this example, in a dark yellow).
All users who participated in the conversation get a notification in the collider.
When the users open the page, they see the suggested deletion (in the form of the comments in question being highlighted, there'd be no extra dialog box or anything) and an option to agree or disagree. Only the involved users see the suggestion.
once all users have agreed, the comments are deleted.
(The UI design isn't great; also the example is suggesting that my comment was in fact worth preserving, which was not really the case. Please regard the contents as the quickly copy & pasted example that it is. You get my point.)
Delete suggestions would wither away after a defined period of time, e.g. 6 or 12 hours.
If a delete suggestion is in place, no new suggestion can be made that overlaps with it. (Therefore maybe the suggestion needs to be visible to the public somehow.)
Even though this puts the responsibility of cleaning up comments into the hands of the users, moderators could still pick up on delete suggestions, and act immediately on the totally obvious ones.
As a rule, only people who participated in a conversation can suggest a deletion, but maybe suggesting deletes on any comment thread could be made a 3k+ or 10k+ privilege.I take this back. The idea is to let participants in a conversation delete their own comments quickly.
This is a bit like picking up litter after meeting in the park. I imagine this could actually be fun to do, and become a habit quickly. A badge could be introduced to make new users familiar with the behaviour.