Thinking a bit more about terminal multiplexing was a major use case for early X, I
recalled using Linux virtual consoles in the late 90’s for this purpose.
According to Wikipedia, virtual consoles originated with Xenix and before that with
concurrent CP/M.
Perusing the documentation of those on Bitsavers, I can see that virtual consoles have a
prominent mention in the manual for concurrent CP/M (1983), but not those of its
forerunners MP/M II and MP/M (1979). I cannot find a mention of virtual consoles in Xenix
documentation as late as 1988.
No such thing as a virtual (as distinct from pseudo) tty on 16-bit Unix or early 32-bit,
as far as I know; one could argue it does not make much sense with physical terminals.
Wikipedia says no such thing existed on SunOS either.
I think virtual consoles where present in Linux from a very early point.
So, as far as I can tell virtual consoles were invented for concurrent CP/M around 1983,
made their way to Xenix in the late 80’s and became part of Linux in the early 90’s.
Have I missed other prior art?