AT&T even had a full OS, where C only had minor role, the microkernel and Dis VM/JIT, with the whole userpace implemented in Limbo, designed by the creators of UNIX and C.
However none of them had an OS available in source code for the symbolic price to send tapes around to universities.
Additionally, the remark that the UNIX and C authors themselves did not stop there, and their latest mark on the computing world was the Go programming language, after Limbo, and the acknowledgement that designing Alef without a GC was a mistake.
Probably they would not had to reboot Plan 9 OS design into C.
As the parent comment indicates, it seems like there might have been more languages that were less known that did try to apply some of these ideas, so maybe I'm misremembering the history that I had heard, or maybe I did remember what I heard right, but the people who originally expressed it were similarly unaware that some of it had been tried already. Hearing more context about the time period I was referring to is definitely interesting, and I appreciate it being shared!