Note that most UNIX was protected by Trade Secret rather than copyright
so there’s no statutory expiration date.
But only so long as they remain secret... there are a few leaks of early stuff in bitsavers... the leader broke the law, perhaps, but once the secret is out there, its no longer secret. See the 32V preliminary rulings for variations on that theme. System V and newer did have copyright protection as well as trade secret contract stuff...
It isn't hard to find at least one copy of most of the major and many minor players. It's fair use to copy for study and commentary. But building a full system for commercial benefit is not.
We were contractors for IBM so we had XENIX and AIX sources in our
facility under NDAs, so I don’t believe IBM dealt with it other than
OCO.
I did have a license for the source code for Interactive Systems IS/1
but that was a fairly straight forward System V kernel.
Several variations on that are out there...
What isn't out there are the full source code control trees. At least Sun distributed them. But so far only a symlink farm for that has been found.
Warner