Hin-Tak Leung authored Jan 20, 2021 and Carlos Garnacho committed Feb 14, 2021

“gnome-fallback” is no longer a valid session name for org.gnome.desktop.session/session-name. It was removed in 2013:

commit bef001f9b0fe41ef731810e6520dcead5bd55334
Author: Matthias Clasen <mclasen@redhat.com>
Date:   Fri Jan 25 16:49:59 2013 -0500

    Remove the fallback session

    We no longer support fallback mode in 3.8, so we don't need this
    session definition anymore. We are keeping the runnable helper,
    since it sets some X properties that we need.

    https://bugzilla.gnome.org/show_bug.cgi?id=687586