There's at least one statement from core: Public classes with private superclass - #9 by John_McCall
It's quite old, but I think the situation didn't change much.
Another aspect that's imho relevant for the proposal is that we don't have abstract classes, and the most common workaround are methods that fail on runtime.
It would at least help in some situations hide the problematic parts, and still expose subclasses which implement all behavior.