0) A system running current Fedora rawhide was recently upgraded to gnome-session-2.91.4. That system doesn't have gnome-shell installed.

1) gnome-session's NEWS files states:
> [...]
>  - two predefined sessions are shipped: GNOME (default), that will start GNOME
>    Shell, and Classic GNOME, that will start gnome-panel, metacity, etc. like in
>    GNOME 2.
>  - a small utility, gnome-session-is-accelerated, is used to determine if GNOME
>    Shell can be used. If this is not the case, Classic GNOME will be used.
>    Please send feedback on when this utility doesn't detect things properly.
> [...]

2) The above doesn't seem to work on that system. It apparently still launches gnome.session, which gives an almost entirely useless desktop (ie, a background, a cursor, and stuff like SELinux warning dialogs).

3) grep gnome-session .xsession-errors:
gnome-session[...]: WARNING: Unable to find provider 'gnome-shell' of required
component 'windowmanager'
gnome-session[...]: WARNING: Unable to find provider 'gnome-shell' of required
component 'panel'

4) copying classic-gnome.session to .config/gnome-session/sessions/gnome.session gave a functional desktop again (using gnome-panel, metacity, etc.)

5) Feel free to ask for more details, etc.

6) Setting severity to major because this can only be fixed (as far as I know) through text based logins (eg, trough ssh or the text console that agetty, login, etc. provide)