When I delete a comment, is it destroyed completely? Or does it still exist, perhaps visible to moderators? If the latter, can it be undeleted again?
In old conversations here, moderators wrote that:
Comments are not destroyed.
The final posted edit of a deleted comment is accessible to moderators.
Some Stackexchange developers can access the complete revision history of every comment.
At that time it was impossible for moderators to undelete comments but that has changed. Since 2013 or so, moderators can undelete moderator-deleted comments and possibly some other categories, but not users' self-deleted comments.
There is a dialog that moderators can open to reveal all of the comments on a thread, whether they are deleted or not, along with other information about the comments (who deleted the comment if it is deleted, who a comment is a reply to if it used an @reply, etc.)
There is (currently) no way to undelete a self-deleted comment (moderator-deleted comments can be undeleted, though), but a moderator can always repost a deleted comment with their own account if necessary.