Yeah, open source Darwin was fairly useless except in very rare situations or for hobbyist poking around.

Open Source Darwin could be useful for Mac OS builds of F/OSS software eg. daemon-like and command-line apps without native GUI.

You can already do that via cross compiling with osxcross[0], but you need the native SDK (you can copy it from any Mac). I have used it at the past as a part of a (Linux hosted) build process to create OSX binaries for some (GUI) stuff i was working on (the same process created Linux and Win32 binaries too).

[0] https://github.com/tpoechtrager/osxcross

Would it be legal to use something like osxcross on public CI service? Yeah I know Travis provide macOS instances, but I still wonder about that.

Also how hard it is to use it with CMake?

Mozilla I believe is now Firefox using Clang on Linux, cross-compiling for macOS against a copy of the SDK, at least for test builds.

Thank you very much for information. I'll look into it since Travis Mac builds sometimes so annoyingly slow so for majority of builds we can just use cross compiler.

And you could already do that with opendarwin before it became broken due to apple not supporting this model any more, and before osxcross existed..