Expurgating phrases like "master" or "slave" from technical jargon does not in any substantive way meaningfully improve the lives of people descended from slaves. Some people forego the use of these phrases from a genuine desire to be welcoming, omitting language that could potentially offend or upset: so be it, they're permitted to do that.
GitHub's house, GitHub's rules. This might impact a few projects which reference "Master" instead of "Principle", "Primary", etc, but it's a small change. Generally speaking I can't see how it significantly impedes anyone's use of their platform, aside from irritating them with faux wokery.