xdg-desktop-portal 1.17.x introduces a new way to select which portals will
be used for which desktop environments, modelled on mimeapps.list:

- each desktop environment should provide a file like
  /usr/share/xdg-desktop-portal/gnome-flashback-portals.conf

- the filename is ${DESKTOP}-portals.conf where ${DESKTOP} is the desktop
  environment's entry in $XDG_CURRENT_DESKTOP (the same as the DesktopNames
  from /usr/share/{x,wayland-}sessions/*.desktop), folded to lower case

- sysadmins and users can override this via files named portals.conf or
  ${DESKTOP}-portals.conf in various locations like /etc/xdg-desktop-portal
  and ~/.config/xdg-desktop-portal

In the case of gnome-session-flashback, XDG_CURRENT_DESKTOP is set to
GNOME-Flashback:GNOME, so in the absence of other configuration it would
use the gnome-portals.conf from gnome-session (if installed). However,
because GNOME Flashback uses metacity rather than GNOME Shell, its mix
of desired portals is probably somewhat different from the full-fat GNOME
session, so probably it should have its own gnome-flashback-portals.conf
that doesn't try to make use of GNOME-Shell- or Mutter-specific interfaces?

https://salsa.debian.org/gnome-team/gnome-session/-/commit/b201c9c40e3adc7bf0b1c3504bef4c8602aac31d
was the equivalent change for gnome-session.

Thanks,
    smcv