Flatpak recommends xdg-desktop-portal-gtk | xdg-desktop-portal-backend

xdg-desktop-portal-backend is a virtual package provided by the GTK
and KDE backends.

Since there is a KDE backend, I think one of the KDE metapackage
should go ahead and install it by default so that a user who installs
Flatpak will get the correct KDE integration without needing to take
the further action of finding and installing the KDE backend.

For reference, the kubuntu-desktop and kubuntu-full metapackages
recommend xdg-desktop-portal-kde.

By the way, Snap will soon be using xdg-desktop-portal too.

Thanks,
Jeremy Bicha