Good call. "Leader" and "follower" may not be the most precise, descriptive terms here, but then neither were "master"/"slave" and people got used to that.
Edit: "Primary"/"replica" seems like an unambiguous improvement.
If you're going to replace a term, it might be a good idea to pick a new one which is precise and descriptive, given the opportunity to do so.
"Senior" and "junior" might work, or "primary" and "secondary". It might be important to capture not just the fact that authority resides with one server and not the other, but that the secondary server is effectively forced to obey via configuration, to distinguish from clusters whereby leaders may be elected, and so on.
Using any anthropomorphic terms at all may end up being misleading - Dijkstra was pretty down on anthropomorphism[1] because he believed that it was a cause of confusion and error. However, if we're going to use anthropomorphic terms then I think it's unwise to choose them based on whether or not those terms, in entirely different circumstances, may have been related to unfairness and harm. The whole point about why slavery is bad is that it involves treating a person like a machine, but this clearly implies that treating machines in such a way is obviously not wrong. Words generally aren't fnords[2] that cause us to become irrationally upset at merely seeing the word itself in print.
"Master/slave is a model of communication where one device or process has unidirectional control over one or more other devices. In some systems a master is elected from a group of eligible devices, with the other devices acting in the role of slaves."[0]
I'd say that master/slave describes perfectly well the relationship between the parts.
Edit: "Primary"/"replica" seems like an unambiguous improvement.