It's worth noting that Stallman had earlier proposed a design for Emacs "to handle all the world's alphabets and word signs" with similar requirements to UTF-8. That was the etc/CHARACTERS file in Emacs 18.59 (1990). The eventual international support implemented in Emacs 20's MULE was based on ISO-2022, which was a reasonable choice at the time, based on earlier Japanese work. (There was actually enough space in the MULE encoding to add UTF-8, but the implementation was always going to be inefficient with the number of bytes at the top of the code space.)

Edit: see https://raw.githubusercontent.com/tsutsui/emacs-18.59-netbsd...