I was going by the fact that tenth edition Unix was released on 1989, while Plan...

smorrow · on July 29, 2023

The first paragraph of https://archive.org/details/rc-shell/

So rc was made for Plan 9. The examples in that paper are definitely on tenth edition Unix, I think this would be because

A fairly complete version of Plan 9 was built in 1987 and 1988, but development was abandoned. In May of 1989 work was begun on a completely new system

from https://doc.cat-v.org/plan_9/1st_edition/designing_plan_9 (1990).

smorrow · on July 25, 2023

Some clues:

The mk paper (and Unix man page) doesn't mention rc.

Plan 9 existed since the mid-80s.

This sentence in the Unix rc(1) sure makes it seem like rc was Plan 9-first:

Environment entries for variables are kludgy for UNIX compatibility.