Do you have an example? Just because it's the word 'slave' and someone might find it abusive isn't a reason for me. Then for me it's just following a sad trend where people try to be overly political correct.
The definition of slave is as follow:
Simple Definition of slave
: someone who is legally owned by another person and is forced to work for that person without pay
: a person who is strongly influenced and controlled by something
I don't think master/slave essentially means the same as primary/secondary. The first indicates clearly that the slave is dependent on the master, where the secondary can be a substitute in the case that the primary isn't existing.
If you want to change terminology because it feels political incorrect (I still disagree it's a reason), I think parent/child or root/node are more suitable, but again, people would probably put a different meaning into this terms.