On the Facebook of the 1990s, Usenet, people routinely posted with their real name/identity and real email address, and message headers showed each server the post passed through from its origin, the IP address of the poster (which could and did result in doxxing) and often an "abuse@ISP" email address to complain about the post/poster.
Remember some of early Usenet, I wouldn't be surprised if some people had taken advantage of Californian naming laws (at the time, one could use almost any name one wanted, as long as it was consistent and not intended for fraud) to make their legal name more consistent with their online handle.
Mark V. Shaney is an example of a user who was not using their real name/identity (nor even had one).
Maybe https://news.ycombinator.com/threads?id=thequux can tell us how pseudonymous usernames were in his time?
Quaxity quuxity,
Teitelman's InterLISP
Has a DWIM feature that's
Really a screw;
MacLISP has evident
Superiority,
Letting its customer
Mean what he do.Moreover, online forums (remember phpBB?) were overwhelmingly pseudonymous on practice.
finger rwj@cxa.dl.ac.uk
and found out who I was all the way through to the mid 90s.(Our undergrad computing lab had donated CAD workstations, so someone came up with a graphical display of the room, showing who was logged in where. The TA was displeased with me after figuring out that I had set my avatar to the "unoccupied" graphic. IIRC, he added a square outline for logged-in workstations to the display, around the same time.)