Regarding #21797 (add VirtualBox to flathub.org / flatpak Integration for easier installation Linux) – Oracle VM VirtualBox stating:
VirtualBox requires kernel drivers and flatpak doesn’t support this currently.
I don’t see any request for kernel module support filed to Flatpak’s issue tracker: Issues · flatpak/flatpak · GitHub returns nothing of relevance. Is there seriously no formal issue debating support for them?