IMHO the only problem is what people wanna see in words.
master/slave has been with us since forever (https://news.ycombinator.com/item?id=23969906) and describes a relationship between two entities where one is the controller and the other is controlled, in many religions (including the most popular one) there is still a master God (or more than one) and believers are slave to God (with the capital G)
But in Git there is simply a "master" branch, there is no master/slave relationship, there is no hierarchy enforced, branches are just copies that diverged from the master copy, master as in master recording or "the source from which all copies will be produced"
I honestly have a hard time understanding what's problematic with it.