David, that is exactly what I meant: functionality for Python 3 is less than the functionality available for Python 2, and behavior is completely out of sync with the documentation.

Bug or not, and independent of the root cause (I don't know if anyone will ever come up with more or better tests), I agree that this behavior for Python 3 has been around for a long time. I don't think it will break any significant code, but that's just my personal opinion; the main point is that re-introducing quoting functionality in imaplib would be a significant effort, also because reading the other relevant threads it is apparent that correct implementation is not there in Python 2 either.

I think that we have an easy way: 1) fix the documentation for Python 3 so that it reflects behavior and 2) expose the _quote() private method with a new static function that the user will be able to call when they deem it necessary.
They get to come up with a good regex or other ways to detect strings that need quoting, but still it's better than relying (like I did) on the advertised functionality and having to dig into the bowels of the module to understand the source of some bizarre bugs...