The plasma-discover-flatpak-backend package isn't very useful without any repos configured. Right now, Flathub is the only really useful one, so I think it makes sense to add it when the package is installed, to save the user (who is probably non-technical if they want to use the Flatpak backend) from having to figure out that they have to add a Flatpak repo, what one would be, how to do it, etc.
I recommended doing this automatically for all distros in https://bugs.kde.org/show_bug.cgi?id=387718, but Discover's author and maintainer advised that this is a distro packager's decision. So here I am, asking Fedora's plasma-discover packagers to automatically add the Flathub repo (https://flathub.org/repo/flathub.flatpakrepo) when plasma-discover-flatpak-backend is installed. :)
This bug appears to have been reported against 'rawhide' during the Fedora 28 development cycle.
Changing version to '28'.
One question is how best to implement that (it is a shame upstream poo poo'd doing it by default).
Probably the simplest approach would be to add a shell script command in the packaging that gets run as the very last step of installing the Flatpak backend package is installed:
flatpak remote-add --if-not-exists flathub https://flathub.org/repo/flathub.flatpakrepo