Availability in the code is not really an issue, we use techniques that can handle it all over the place in Kotlin (there’s name mangling, and there’s synthetic accessors).
The only issue is that normally programmers can change/remove private declarations without affecting public APIs, and if such declarations are in any way exposed (e.g. through inlining), there will be unexpected breaking of APIs. This is why I said all we need is a modifier/annotation to mark exposed private functions explicitly: the API owner will see them, and take measures.