I had never heard of them either until Billy started going on about them. Apparently it's some Instagram/Vlogger thing that started around 2010 and is only really a thing among the kind of social media types that are into this entire "share your entire life with the Internet" stuff. So my guess is it's mostly a thing among whiny SJWs because whiny SJWs and social media go hand in hand.
Kinda curious to know what Billy's reaction would be to learning that most people would never consider throwing a gender reveal party - instead they'd just tell people if they're having a boy or a girl.