Relevant place to ask: I’ve been trying to find a reference for the earliest Emacs that could host a terminal emulator or subshell in a window.

Multics emacs appears to have had both split windows and a character-at-a-time input and output mode as far back as 1978 for use as a SUPDUP and/or TELNET client, which is currently the earliest I’m aware of. Ancient ITS TECO EMACS had splits pretty early on, and may have sprung the necessary character plumbing earlier - but I’ve never found any reference material to confirm/deny.

It’s a fringe to a larger interest, which is that I’ve been trying to document the history of terminal multiplexers, especially in the Window (1986)-Screen(1987)-Tmux(2007) tradition (as opposed to the historical meaning which we’d call terminal servers). I’m slowly becoming convinced they came about after the advent of floating window GUIs hosting multiple terminal emulators. If you were super connected and could get access to one, sometime fairly early in the window between the 1973 introduction of the Alto and the surviving 1979 manuals the Alto program “Chat” could run multiple telnet sessions in floating windows (I’m also looking for a more precise date for when Bob Sproull made Chat able to do that trick). Several other early graphical systems like Blit terminals (1982 inside Bell, commercial as the 5620 in 1984) and early Sun Windowing System of early SunOS (1983) could also do multiple floating terminal emulators, so they were common by the early 80s.

Because the 36-bit DEC lineage had pretty robust psuedoterminals all the way back into the mid 1960s ref, a lot of hackers did a lot of fun shit on PDP-10s with ITS and TENEX and WAITS, and Stanford and MIT had PDP-10s connected to fancy video terminals by the mid 70s, it’s IMO the most likely place for the first terminal multiplexers to emerge… if I could just find some documentation or dated code or accounts.