While X Input 2 (Xi2) was around much longer, it was officially released with X Server 1.7 in October of last year while it was merged into the mainline X.Org Server some months earlier. X Input 2.0 is a critical piece of Multi-Pointer X (MPX) support, also developed by Peter Hutterer, for handling multiple cursor inputs simultaneously.
Confirmation of X Input 2.0 support being merged into the mainline GTK+ repository can be found in this GNOME bug report. For some months prior this Xi2 support had lived in a branch of GTK+. Information on Multi-Pointer X support for GTK+ can be found on this GNOME Live Wiki page.
This support can now be found in GTK+ 2.90.1 (a GTK+ 3.0 development snapshot) that was just released over the night and this GTK+ update can be read about via its release announcement.