I'm okay with breaking forward compatibility in internal/impl packages when a bugfix is impossible otherwise.
It would still be good to ensure these methods won't (accidentally) be called by users, as that would create a dependency on this particular version of Scala. So, could you mark this method `protected[scala.concurrent]`? It's still against the letter of the law, but in the right spirit :-) (Perhaps we should make MiMa more flexible to take scala visibility into account when checking forward compatibility. Even better, generate the right access modifiers in bytecode so Java clients are protected as well.)