Is there a way to curry this:

suspend fun Flow<AllStates>.assertTransition(oldState: AllStates, newState: AllStates) {
    ...
}
// into (by providing the oldState argument...):
suspend fun Flow<AllStates>.assertTransition(newStates: AllStates) {
    ...
}

Just use a lambda:

val assertTransitionCurried: suspend Flow<AllStates>.(AllStates) -> Unit = { assertTransition(it) }

Yeah, that's probably simpler in this case... 😊. I wonder where all these partials and currieds really come to use then?

They come to use when there's a simple case. Keep in mind though that Kotlin, by design, doesn't work well with function composition and transformations like that. The "Kotlin way" is to use lambdas to transform the shape of functions