Isn't that the same thing that Homebrew does? It only has CI for the last couple of Mac OS X versions. Seems like this is more of an issue with Mac OS X architecture than Go.

Dunno about Homebrew (haven't used that since many years for various reasons), but Apple is certainly pushing things in a way that makes 3rd party developers quickly abandon old systems too. That's true. At least lots of 3rd party developers are very quick to give up if their new Xcode will not cooperate.