I consider to switch to the new multiplatform gradle plugin. Is it possible to specify compileOnly and runtimeOnly dependencies? So far I have only seen api and implementation. But maybe I don't understand ap/implementation correctly

Yes, and it works the same way as api/implementation IIRC

It is possible: https://kotlinlang.org/docs/reference/building-mpp-with-gradle.html#adding-dependencies

Thx for the clarification -- I was exactly on this page but didn't spot it. I must have been blind 😀