In the wider context of computing and technology, "master" has historically often been paired with "slave" as well, such as old IDE hard drives that had to be switched from master to slave depending on which cable they were plugged into on which port of the motherboard. I realise that's a bit of an odd example, but there are numerous ones.

Anyway, while I don't think many people have ever used a branch name of slave, it's entirely feasible to argue that any branch that isn't master is in some way subservient to it as opposed to the master branch being the most experienced. The point isn't to debate that the way you view it is incorrect, your view is entirely reasonable and rational but in order to be inclusive we should take all other views into account and in a very simple way, rather than debate the meaning of the word master in this specific context and telling people that they're "wrong" for feeling a certain way about it, it's easy to change the word and thanks to the excellent design of git, there isn't really any downsides to it.