I think the idea of "master" as in a definitive or original copy has enough usage that it's considered a second meaning of the word. I guess it's still possible that people will be upset, but without "slave", it's not unreasonable to assume that people will think of a key or album master instead of a slaver.
Is master/slave even a proper term to begin with? If we approach it from a "human slavery" standpoint, the master directs slaves to perform tasks that the master does not itself perform.
Master/replica makes sense to me in the context of copies that all perform the same function, including the master.
Even that doesn't quite fit, I think. It makes sense for Redis, but in a lot of other master/slave topologies, what's actually happening is that 1. a group is electing a leader; 2. the leader says what is happening; and then 3. everyone (including the leader) all does what the leader said, concurrently.
I feel like there is probably an exact term for this arrangement, from another discipline. Maybe choreography?
• There are principal dancers and back-up dancers. People performing the same action are dancing in unison.
• There are cheerleading captains and their squads, which consist of the roles of base, flyer, and spotter. Interestingly, being captain (giving orders) is disjoint from a member's performative role within the squad; a member of any role could also be the captain.
• There are operatic prima donnas (or primo uomos)—the lead singers—and a chorus, though these are not mirrored.
• Orchestral organization has tons of different roles: the orchestral ensemble composed of players; their conductor; the principal of each section; the concertmaster (the principal of the section leading the melody of the piece, usually the first violin section.) You can also speak of accompaniment.
Interestingly, most of these disciplines lack a term for "everyone of a group being led, besides the leader." I would assume that this is because the leader is taking part in the group, and so there is no explicit action taken by the group-minus-the-lead, only actions taken by the lead or taken by the group as a whole. (On the other hand, for machines, it is useful to talk about the process of following the lead's... lead. Humans generally do this implicitly, and don't really refer to it as an explicit action they're undertaking, separately from the actions that they're mirroring.)
Now this is the kind of argument that could actually persuade any remaining holdouts. It's not bad terminology because it's offensive but because it's inaccurate!
Personally I wasn't-necessarily-a hold out, rather dispassionately observing from the sidelines but I'll admit that approach to the descriptors made me subtly. It makes the most sense, and much more accurately defines the mechanisms involved.