Originally, self-deletion of comments was not throttled.
Instead, an automatic flag was raised when a number of comments were deleted in a short time.
This flag was shown to moderators and displayed in the 10k tools.
The flag was rare and not super-effective because even moderators cannot undelete user-deleted comments.
In 2013 the system changed to flag for moderators only, when upvoted comments on 20 posts were deleted.
The implementation was somewhat broken for a couple of years, but fixed in 2016.
In a comment on that answer, Shog9 said:
In the entire history of Stack Overflow, where millions of users have posted millions of comments, there are only a few dozen instances where anyone has deleted a significant number of upvoted comments in a day.
The only reason we even have a check for this is one time someone had to undelete a whole lot of comments and hated it.