Except, you know, apprentices eventually become masters themselves. And the space isn't zero-sum; "Master Verrochio taught Master Leonardo" is semantically valid. It doesn't illustrate the semantics of, say, data replication where there is only one master at a time and the slaves are not supposed to be promoted. "Master-slave" captures the relationship succinctly.
True, it hearkens back to a dark time in US history but if that is the only criteria for censoring usage of this term, are we not also censoring discussions about this period? I agree with the sentiment to be ashamed of it but to the point where you can't even derive metaphors from it? Reeks of cowardly denial.