They are not part of the API, that is why they are not documented.  The convention of "always" using _ prefixed names for non-API stuff is (relatively) recent.  It used to be we just didn't document the non-API stuff.

Your second argument is a good motivation.  Let's see what others think.

I thought the deprecation process was documented in a PEP, but I can't find it.  Basically, we introduce a deprecation warning (and a document the deprecation, but that isn't needed here) in the next release, and then the release after that we actually do the removal.  Or in many cases we don't do the removal at all, we just leave the deprecation warning in place until 2.7 is out of maintenance (but I don't think we need to worry about that in this case).