This subrepository holds the source for various packages and tools that support Go's build system and the development of the Go programming language.
As far as I know, the x/build repo is different from all other golang.org/x repos in that it's meant to serve our internal needs and does not offer stability guarantees the way other golang.org/x repos do. For instance, we did not have CI for this repo for a long time and it was okay. When we started testing this repo in CL 157438, it was at a time that was convenient for us and only in module mode without any support for GOPATH mode. There was also discussion of the target audience of x/build in issue #29935.
Do you know if this is documented anywhere @bradfitz? I think I know this intuitively, but I'd like to be able to point people at something that is written down. Also, if it's not documented, then Go users can't know this.