When installing the Flatpak backend from inside Discover, it would be nice if flatpak-kcm was installed automatically too.
The flatpak backend is part of plasma and expected to be installed. If not that's a distribution problem.
Kubuntu doesn't ship flatpak by default (for obvious reasons). The discover flatpak backend is in a separate package and also not installed by default. But it can be installed comfortably from inside discover (packagekit?) and it would be nice it that triggered also the installation of flatpak-kcm.