State of immutable collections in Kotlin?

I’m looking for news about the current state of the mutable collections approach in Kotlin.
kotlinx.collections.immutable seems to be frozen for about a year and all I read is “there are plans…”.
Is there any issue / discussion I could follow?

Take a look at the develop branch. The last commit was in June this year. I don’t know about any active discussions about this outside of the projects issues though.