Which Arch-based distributions ship with Flatpak installed by default? I am always left with the impression that most Arch users prefer binaries and not the overhead of yet another package management system.
However–a typical Arch KDE installation, either plasma or plasma-meta, does include Flatpak by default.
You can very easily install Flatpak in KDE, just $ pacman -S flatpak flatpak-kcm. Same with Snapd or Appimage.
You don’t need to let a developer’s design stand in your way.