I don't recall ever seeing "open source" used as a description of the Unix "ecosystem" during the 90s.
Yes, that's my point. The term 'open' meant was published, open available anyone could use .. i.e. UNIX
remember the SPEC 1170 work on the 1990s -- the define the 1170 interfaces and >>publish<< them so anyone could write code to it.
It was in the air with the (minimal) charges Prentice-Hall charged for
the Minix 0.x and 1.x disks and source; not dissimilar in that sense
to the charges the FSF were charging for their tapes at the time.
Right... there were fees to write magtapes (or floppies)
Which comes back to my point... 'open source' was not picking. The whole community is standing on the shoulders of the UNIX ecosystem that really started to take off in the 1970s and 1980s. But the 'free' part was even before UNIX.
We stood on the shoulders of things before us. There just was not (yet) a name for what we were doing.
As Ted saids, I'll give the Debian folks credit for naming it, but the idea really, really goes back to the early manufacturers and the early community.
FSF was a reaction to the manufacturers taking away something that some people thought was their 'birth right.'