I really like this idea and do not understand why it received so little attention.
At the moment SO lacks documentation which makes users aware that posting comments also leads to some kind of responsibility for these comments. Especially there is no real hint in the documentation that comments may often become obsolete after some time, which potentially leads to additional work for the community and moderators who need to flag/delete these obsolete comments. Obsolete comments are only briefly mentioned on the "What if I see someone doing something bad?" help page (a horrible title btw) under "Flagging a comment". (Actually, even though I did read the entire help when I got a member here, and even though I did read quite a lot of the meta discussions, I only recently became really aware of the possibility to flag comments as obsolete.)
In my opinion adding a badge for deleting comments could greatly improve this situation by showing the users early on what is considered good commenting practice. In addition to the badge there should also be some documentation added to the Commentator badge, which encourages users to look through old comments now and then and delete obsolete ones - at the same time mentioning the new badge.
I would even go a bit further with this badge request and not reward it for deleting a single comment but instead for deleting say 10-30(?) comments which could also considerably reduce the number of existing obsolete comments on SO. If only 1000 users would delete 10 obsolete comments, this would mean 10000 fewer obsolete comments to handle for the community and moderators!
But as argued in the comments here, there would need to be some constraints for such a badge, because otherwise it would be too easy to cheat.
These are some suggestions:
- Deleted comments only count toward the badge, if one currently has a certain minimum number of comments (20-30?).
- Only comments deleted after a certain time count (a month?).
- Only comments with fewer than 2(?) up-votes count.
I believe constraints like these would greatly reduce the chance, that the system gets cheated or that actually useful comments get deleted just to get the badge - discuss...
Some interesting discussions about obsolete comments in general: