I’ve been semi-remembering a dinosaur remote access feature from the days when you couldn’t guarantee an 8-bit path between client and server at all times, where you could manage a login to your mixed-case username/password, if you entered the initial username in all CAPS.
Googling for “unix login uppercase” just gave me a bunch of pages encouraging people to make passwords that mixed upper and lowercase letters with numbers and symbols. Good advice, but not what I was looking for. Then our security guru here at work suggested replacing uppercase with 7-bit, and sure enough I was able to find documentation that something like my memory existed, at least in aix 4.
I’m still not entirely convinced that an admin couldn’t enable this feature on a modern system somehow. I guess the program “adduser” would be an easy place to look, since it would have to enforce case-free uniqueness.