What about having a plone.api v2 which is AT free for Plone 6 and above?
A big plus if plone.api v2 should support 5.2 without AT (i.e. travis build should pass) but the default version shipped with Plone 5.2 should still be plone.api 1.x.
That would cover all the current versions that work with Python 3.
If somebody wants to use plone.api 2.x with Plone 5.2 he just has to add a pin to the buildout.
If there are still AT users out there they can stay with plone.api 1.x or do the extra effort to backport the changes to work with AT.
I would not even put in the game Plone versions < 5.2.
Another option is to go the hard way and also remove completely the support for Python2 for plone.api v2.
But that might be too early, see https://plone.org/download/release-schedule.
Open for discussion