> I would love to see a deep dive on what features / architectural paradigms the Golang runtime shares with Plan9. Has anything like that been written?

If it has, then it's most likely available on https://cat-v.org/. Even if it hasn't, cat-v.org is a great starting point.

Besides, close to your line of thought, and assuming you didn't knew about this already, Pike & al previously worked on Limbo[0], a "predecessor" of Go, used to wrote Inferno[1], a Plan9-like OS, which could be hosted on arbitrary OSes via a bespoke virtual machine called "Dis".

So there were indeed a few previous "drafts" for Go. I'd doubt that Go has been designed "for" Plan9 though.

[0]: https://en.wikipedia.org/wiki/Limbo_(programming_language)

[1]: https://en.wikipedia.org/wiki/Inferno_(operating_system)