I see people claim this frequently that Kotlin is used a lot on the backend, but there’s very little evidence that this is true.
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.
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 don't see Swift anywhere outside of Apple and don't see why anyone would reach for it instead of Kotlin.
It also doesn't help that JetBrains has an IDE that makes development in the language so nice people are willing to pay for it and Xcode easily is the worst user experience of any current product developed by Apple.