We had a bunch of Kotlin on the server at Airbnb in 2018.
Nope, Kotlin is plenty popular on backend, so much so it's mundane to see it in popular frameworks like Spring and Play. It helps that it's designed to coexist with Java.
It is almost meaningless, about 10% if at all.
I absolutely will concede that Swift is in practice an Apple only language, but I feel Kotlin is essentially an Android only language in practice as well.