Everything I read around the web says "don't use it, it could break any time because it's private, etc.". I agree that it could break any time, but that annotation is widely used in some Swift communities and I think a hue and cry would be raised should it be removed.
I found this thread that discusses this subject of the visibility of an import and is also the only place I have seen a Core Swift Team member discuss it. From what I read, I don't think there is too much of a concern at this point of its removal, though I could be wrong on that.