Seems rather unlikely considering that UTF-8 was originally invented for Plan 9?

Turns out it is the same as anywhere else in Plan 9 - tap ALT (not hold) then the letter x then the hex code for the rune you are trying to enter and a semicolon (if fewer than 6 hexadecimal nybbles).

Example:  ALT x 2 1 d 4 ;

There are shorter versions for some characters - see the man page keyboard (6) for details

I was under the impression you could only use ASCII.