Dollar to a donut, there is or was a mainframe involved in authentication that has an 8 character password. I even bet the company that sold it was IBM. I remember finding out at one job that the "reason" for the 8 character password requirement for one of our logins, was "you can type as much as you want, but eventually we just take the first 8 characters and use that to log you into the mainframe as thats all it accepts".
I remember DB2 had some weird limits. I think it was something like database names couldn't be more than 8 bytes, which was something to do with some filesystem on z/OS or something that couldn't have directories with names longer than that (probably not quite right, but it was something like that).
z/os, oh how I remember interfacing with thee, and hating every minute of it.
From what I remember you're spot on, every "file" or "record name"/whatever (mainframes are not unix) was restricted to 8 characters and upper case, also EBCIDIC just because IBM. So many silly legacy things around those mainframes.
That said, they literally never had an outage on it the entire time I was there so +1 for reliability and availability. They even replaced the whole mainframe piece by piece.
I’ve seen limitations that even go back to paper records. I worked at a medium sized company where at least 20% of database variables were limited by the number of characters on a paper card system developed in the 50s.
Mainframes are weird, and not all that dynamic.