For me it really isn't about the literal change in words. This matters more about the why github is doing it and the effects of them preforming said action.
It is an unnecessary change that, in reality, is only being preformed because the bubbles in the boiling pot that America is started flowing over the brim.
It doesn't effect or reduce police brutality, nor racism on the internet and on the streets. It doesn't change white on black or black on black violence, or actually any violence.
The change isn't because it would only help developers (that use github) as a whole, but because it may bring good press, or press at all. And it has.
Depending on implementation (if the default changes and 'master' aren't kept as an alias as mentioned by others), they break scripts. Lots of 'em.
Please understand that this is my pov, and I hope this explains why I thought it was worth my time to put it over here. It may just be extremely poor timing on GitHub's part. I really do apologise if this comes through as an emotionally charged reply.