Depends on what you mean by "OS support".
Early Unix (and all other OS) used video terminals that replaced the teletypes of even earlier Unixes.
These video terminals started supporting graphics and a bit later also color graphics. The first color graphics video terminal from DEC was the VT241 with Regis and Sixel graphics from 1983. BTW, Sixel graphics still work in many Linux terminal windows on modern systems.
An early color graphics IBM video terminal (non-Unix) is the IBM 3179G from 1984.
I am sure there were others.
These graphics worked by sending with special escaped character sequences, so any "OS support" (if present at all) consisted in libraries that applications could use.
The X Window System that became the standard for Linux appeared in 1984 (and was probably not very usable right then), The X11 protocal variant that it finally settled on was defined in 1987.
Unix v6 is a lot earlier than all of that. Of course you could still compile and run applications on your Simh-emulated Unix v6 that then send Sixel codes to the
xterm you've attached to Simh. Just don't expect any GUI.