I have been trying to sort out a checkuser/oversight case, and noticed I've been greatly hampered by the "hide username" function. While this is extremely valuable, it also makes it near impossible to track down edits of a disruptive user, which is precisely the kind of work oversighters and checkusers often have to do.
(More generally, in the past it's been possible for anyone to track a change such as a rename, move, oversighting, or deletion in the logs and history provided they are able to see the source entries (ie have the appropriate admin/oversight rights needed). We need to not lose that ability with these new tools.)
If the user viewing material is an oversighter or otherwise would be allowed to see or modify the "hide username" setting, then the username appears as "[deleted] (show/hide)" or "[hidden] (show/hide)" in lists and other places, not just be omitted.