The headline is inflammatory and misleading. IIUC, flatpak is not depending on systemd as a whole, they're depending on a new component called systemd-appd, which does permissions stuff. This will allow alternate implementations, like how elogind grew out of systemd-logind.
IOW, it is an API + a spec, just like what you're asking for. But it's implementation first, because any API + spec designed by committee rather than being driven by an implementation universally sucks.
IOW, it is an API + a spec, just like what you're asking for. But it's implementation first, because any API + spec designed by committee rather than being driven by an implementation universally sucks.