It is available. At least https://build.golang.org/ lists plan9/arm as one of builders. Interestingly enough, the source code build help page[1] doesn't list plan9/arm as a valid build target.

EDIT: There is an open issue[2] about updating that page.

[1] https://golang.org/doc/install/source

[2] https://github.com/golang/go/issues/28142