But git doesn't use master/slave (noun). It uses master as an adjective ("master branch"). Think cassettes and vinyl records or "master boot record". It means being the definitive source of truth and has been used in that form for centuries [citation needed, but I don't remember where I read it].

This isn't just an instance of "not old enough", it's an instance of it not even being the same word, so I guess you just can't win this...