I think it would be safe to assume, that there are many scripts floating around, that rely on the existence of master branch. Its like port 80 for http - you don't mention it, cause its never going to change, right?
In any case, those who are offended need to grow up. No one thought about black slaves when calling the main branch master.
Sometime a word is just a word.
Also, I don't understand why people are offended by it.
I'm Jewish, if someone will call their application SS (for good reasons) I will not be offended. When people are talking about a gas chamber (when discussing SpaceX's engines of internal combustion engines etc) - I'm not offended. I just understand - this is the term used, it fits, it has nothing to do with me or with my history. That's all.