Undocumented in source.
> GtkUIManager is deprecated since GTK+ 3.10. To construct user interfaces > from XML definitions, you should use gtk.Builder, gio.MenuModel, et al. To > work with actions, use gtk.Action, GtkActionable et al. These newer classes > support richer functionality and integration with various desktop shells. > It should be possible to migrate most/all functionality from GtkUIManager.