GNOME Flashback is a session for GNOME 3 which was initially called “GNOME Fallback”, and shipped as a stand-alone session in Debian and Ubuntu. It provides a similar user experience to the GNOME 2.x series sessions. The differences to the MATE project is that GNOME Flashback uses GTK+ 3 and tries to follow the current GNOME development by integrating recent changes of the GNOME libraries. The development currently lags behind a little but a lot of progress has been made and most importantly many open bugs have been fixed.
No desktop environment truly ever dies.