The GTK+ 2.90.4 development release now enables GSEAL by default, moves gdk-pixbuf into a separate module, renames GtkExtendedLayout to GtkSizeRequest, gtk_init now issues a warning of GTK2 and GTK3 are being used in the same process, theres a few additions to the API, many bugs have been fixed, and there are some translation updates. GSEAL is the macro to assist developers in migrating from GTK2 to GTK3 by ensuring there are no direct calls to object fields but rather helper functions are used.
The GTK+ 2.90.4 release announcement with the full change-log and the ten bugs that were fixed in this release can be found on the GNOME mailing list.
GTK+ 2.21.3 was also released today with similar changes to the GTK+ 2.90.4 release. Again, GTK+ 2.22 is the last planned GTK2 tool-kit release, it will work in parallel with GTK3, and it's not receiving any major new work besides additions that are required in porting to GTK3. This release announcement is available here.