As a mysql DBA, this terminology issue made me uncomfortable for most of the past 15 years. I'm uncertain whether I agree that it's relevant in GitHub terms since we don't have upstream/master -> fork/slave, but it's inspired me to think about the term "master" in the mysql, git, and art proofing spaces simultaneously, so I'm glad they're making the change and I look forward to finding out what I think about it after some time has passed.
Databases have been slowly shifting to other terms for a while now. I.e. consensus algorithms typically speak about "leader election", replication systems use "primary"/"replica", ...
