Within the last few months, gnome-flashback-xmonad stopped starting
gsd-media-keys when I log in.  Even when I start gsd-media-keys
manually, it fails to adjust screen brightness.  This turns out to fix
it:

    cd /usr/lib/systemd/user
    sudo cp -a gnome-session@gnome-flashback-metacity.target.d
gnome-session-x11@gnome-flashback-xmonad.target.d

The file /usr/lib/systemd/user/gnome-session@gnome-flashback-metacity.target.d/session.conf
is provided by the gnome-session-flashback package, whose changelog
for gnome-flashback 3.38.0-1 says "Update
debian/gnome-session-flashback.install for new systemd session
configuration."