Starting with GTK+ 3.9.8, the GtkStock APIs will be removed from the source code of the project. However, they will continue to work until GTK+ 4 is officially released. At the moment, the following APIs have been already deprecated in this release: GtkIconFactory, GtkIconSource, GtkIconSet, GtkImageMenuItem, GtkUIManager, and GtkAction.
Moreover, as discussed at the recent GTK+ hackfest, the GtkSettings cleanup is now in place and a number of settings have already been removed from GTK+ 3.9.8, like gtk-button-images, gtk-show-unicode-menu, gtk-show-input-method-menu, gtk-enable-mnemonics, gtk-auto-mnemonics, gtk-color-palette, gtk-can-change-accels, cursor blink settings.
GTK+ 3.9.8 also adds support for scaled output on high-dpi screens, which can be compiled using Cairo 1.12 (needs a Cairo branch to work). Support for this has been added to the Wayland, Windows and Mac OS X backends.