Exactly. It would simplify the documentation to simply deprecate everything but .on() but would that justify breaking all the existing code out there? Not to me. The old APIs need to stay around so that code continues to work.
Even if they excised the old methods from the code, hopefully no one is linking to jQuery trunk or something as their primary library. The old versions would still be around. Just some simple work might have to be done to update a web app to using the latest versions of jQuery.
They could deprecate the old methods and still keep them around as long as makes sense. There's no rule that deprecated methods have to be gotten rid of during the next major revision or anything.