I think this is a good example of why the pragmatic version of “Don’t use private APIs” is “Be aware of the risks of using private APIs”. Any feature that provides some kind of compile time check on private API usage should have an escape hatch, because there are situations where being locked to a specific version of the language and having a feature is sensibly considered a worthwhile trade off.
A tracking mechanism for private usage would actually make that sort of thing safer, since it’d be easier to audit.
