Since Apple seems forever stuck on an ancient Bash version (due to GPL 3, I believe), I keep hoping they switch to Fish (or a home built solution, à la Swift, which could be interesting) with every OS release.
I remember installing and using fish 4 or 5 years ago; it was not a great experience -- but after he "took over", I switched and couldn't be happier with fish.
So if there is such a policy at Apple, I would expect that it doesn't prohibit contributing to already-public projects.