What I don't understand is, why did they have to keep <IE9 support in version 1.9 AND break compatibility by removing a bunch of functions as well as create a new version strand, 2.0, that drops support for <IE9. Why not jus break the API in version 2.0 only?
They've specifically mentioned a few times being able to detect browser version and load the appropriate version of jQuery. The 2.0 branch is supposed to get cleaner, faster code by virtue of dropping old workarounds (I imagine in some instances this may be quite a difference, as they may be able to refactor entire code paths if they can make new assumptions).
As such, they need to keep the API standard between versions. Unfortunately, they don't appear to have settled on the best API yet, so are constantly working to improve it.
I doubt the functions you are seeing removed[1] from 1.9 are specific to 1.9 being special or dual released with 2.0. They were more likely just removed because they were at the end of their deprecation cycle.
