Hi Ludo,

https://github.com/GNOME/gdm/commit/0bb8a777cfc0a3bc53c2c2830dd7e6e2baeeef38#diff-9b802b1ffb5f0ff95bfaa85046d262e7 says:

>custom is a magic name we used to allow for having the user decide their session by a ~/.xsession file.  We no longer support that construct and haven't for many years. Instead, users who want that functionality can just add a xsession file to accomplish it.
>This commit just removes some left over cruft that never got cleaned up.

Another part is still there:

if [ "x$command" = "xdefault" ] ; then
  if [ -x "$HOME/.Xclients" ]; then
    command="$HOME/.Xclients"
  elif [ -x /etc/X11/xinit/Xclients ]; then
    command="/etc/X11/xinit/Xclients"
  elif [ -x /etc/X11/Xclients ]; then
    command="/etc/X11/Xclients"
  else
    if [ -n "$zenity" ] ; then
        disptext=`gettextfunc "System has no Xclients file, so starting a failsafe xterm session.  Windows will have focus only if the mouse pointer is above them.  To get out of this mode type 'exit' in the window."`
      "$zenity" --info --text "$disptext"
    else
      echo "$0: Cannot find Xclients"
    fi
    exec xterm -geometry 80x24+0+0
  fi
fi

... but not sure how to use it.