I am told by the GNOME/Flatpak people that this is what we will
ultimately need to implement. Basically this portal is supposed to work
everywhere, in sandboxes (Flatpak, hopefully Snap too?), but also out
of sandboxes, i.e. in GNOME and KDE, whether Wayland or X11. So that
should be the unique API we will have to implement in the end, and every
desktop environment/sandbox will need to implement this API (which is
good!).
Apparently it is not part of default GNOME yet, but has to be installed
separately (on Fedora, package is xdg-desktop-portal-gtk for GNOME and
xdg-desktop-portal-kde for KDE).

Now there are currently many shortcomings, and in particular, the
screenshot API has apparently no advanced features (at all!). No window
snap, no rectangular selection, no delaying, no choice on including
cursor or decoration, nothing! Apparently this is normal that the API
presents no feature, because "the API itself is not meant to specify the
details how the screenshot is obtained. Instead the portal will present
the user a dialog to take a screenshot, and that screenshot will be
given back to the sandboxed app".
This is acceptable behavior, except that currently, the dialog has none
of the basic features so this is a very bad regression. This is why I
test the freedesktop API last, which basically means it will likely
never be actually used. That's on purpose. At least, the code is in and
will be easy to improve later. Of course, when the Freedesktop portal
for screenshot will finally be featureful, it is meant to be tested
first.

See: https://github.com/flatpak/xdg-desktop-portal/blob/master/data/org.freedesktop.portal.Screenshot.xml