> The article almost makes it sound like UTF-8 was a Plan9 invention.
It more or less was: https://en.wikipedia.org/wiki/UTF-8#History
It more or less was: https://en.wikipedia.org/wiki/UTF-8#History
But Plan9 didn't influence the design of UTF-8 only in as far as having a better character encoding than the original 2 byte UCS encoding.
And then Plan9 was used to showcase UTF-8.