Backwards compatibility would already have been broken when we refactored the magics into separate classes for 0.13, and I don't remember hearing about any problems, so I don't think it's an issue. We can add a note to the "what's new" document in case anyone is using it.

The API docs unfortunately default to documenting every class and function, without us ever having to consider what constitutes our public API. I've added an `@undoc` decorator to explicitly mark things as private, but I'm reluctant to apply it to magic classes, because at present the API docs are the only fully comprehensive documentation of the magic commands we offer.