The fallback mode, also known as the classic GNOME session, will no longer be included starting with GNOME 3.8, the next stable GNOME release. The GNOME 3.8 features page mentions some of the reasons behind the decision to drop the fallback mode of which, the most important one seems that “it’s in maintenance mode”, without no new features or active development for a long time. Dropping the follback mode doesn’t just mean there will be no more classic GNOME session. With it, some GNOME modules might go away, like Metacity, GNOME Panel, GNOME Applets, notification-daemon, GNOME Screensaver, polkit-gnome and nm-applet. Further more, this decision affects Unity, LXDE or Xfce too because they make use of some fallback components like tray icons of GNOME Settings Daemon.