Anectodal, but from what I've seen spending a couple of weeks at a Chinese mobile games company in 2017, Golang game servers are quite popular in China but not elsewhere (in combination with Unity for the game client and Protobuf for the message protocol). I have no idea why that specific tech combo is "big in China" but not outside.

AFAIK in the rest of the world, Photon Server is the most popular off-the-shelf solution (not necessarily for true MMOs though). Maybe that's good enough for most games to not wanting to write your own server backend (https://www.photonengine.com/)

I actually am building an MMO server for an old MMO. And I am using Go and Goroutines for it. I have a small rundown posted here in this thread: https://news.ycombinator.com/item?id=37972638