Well, since two of the original three developers (Pike and Thompson) were part of the Plan 9 team, it makes perfect sense that they based the compilers on the Plan 9 ones.

I see recent commits in Go's mercurial history to fix bugs on Plan 9. Is anyone at Google actually using Plan 9 to develop Go? Or is building for Plan 9 just a sanity test for maintaining cross platform code?

Wikipedia says the last "stable" release of Plan 9 was in 2002, but there seem to be a number of recent forks.

Plan 9 is on a rolling release since 2002.

There are people that don't work at Google who contribute to Go. It's an open source project.