The first I heard about it 17 years ago, in news articles about "a black employee of the county’s Probation Department filed a discrimination complaint with the Office of Affirmative Action Compliance after spotting “master” and “slave” labels on a videotape machine." - https://www.snopes.com/fact-check/masterslave/ .

Projects started changing terminology at least 6 years ago, like Django at https://news.ycombinator.com/item?id=7801646 .