I realized something curious recently: that the AJAX backend feature that lets moderators view deleted comments also works for any users with the "access to moderator tools" privilege (10k+ rep on graduated sites, 4k+ on beta sites). There's no official user interface to it (that I know of), but it only takes a couple of lines of jQuery to write one.
My question to the SE folks is, is this a bug or a feature?
According to this old answer, only ♦ mods can view deleted comments. However, that answer was originally written back in 2009, before anyone could view deleted comments, and was updated in 2012 when the ability was added to the moderator interface, so I'm not sure if it can be taken as a 100% reliable and up-to-date statement that this is precisely the way it should be.
If this ability is an unintentional security loophole, I hope it will be fixed quickly. If it's an intentional (but undocumented) feature, I have a nice little user script I can post to make it a bit more accessible. ;-)
Ps. Some notes about the script:
Yes, it shows both mod-deleted and self-deleted comments.
It does require the "access to moderator tools" privilege; otherwise the AJAX call will just fail. For example, it doesn't work for me here on MSO, since I don't have 10k here yet.
It does not appear to be possible for non-mods to undelete comments, not even those they've posted and deleted themselves.
The main limitation is that, in order to view deleted comments on a post, you first have to guess that there might be some. (Technically, the script could try to automatically load deleted comments for every viewed post in the background, but that would be incredibly obnoxious and bandwidth-wasting behavior.)
Pps. Just to show that it works, here's a screenshot from the comments to the highest-voted question on SO, showing a couple of mod-deleted comments (with non-deleted replies) and one self-deleted one:
(Actual text of the deleted comments pixelated, just in case.)
Personally, based on the limited testing I've done, I've found the script quite useful in making sense of long comment threads that may include partially deleted conversations, like the one shown above. That said, while I'd be sad to see this ability removed, I can certainly live without it, if it turns out to be an unintentional loophole.