It has arrived. Kotlin version 1.1.50 was released and it has all the extension points that enables the prototype of Kotlin serialization to work both on JVM and on JS. The usage of the prototype compiler plugin and runtime library is explained here: GitHub - Kotlin/kotlinx.serialization: Kotlin multiplatform / multi-format serialization
It comes with JSON, CBOR and Protobuf support out of the box. More to come. Most of the planned use-cases for serialization of Kotlin classes are supported with a notable exception on generic user-defined classes, but all the standard collections are fully supported in a type-safe way.
Your feedback is welcome. Please, submit your issues and suggestions directly to the GitHub project issues.