If you visit the Flatpak website today, it lists, as the very first advantage of the project: “Build for every distro: create one app and distribute it to the entire Linux desktop market.” If you then move on to the list of supported distributions, you’ll see the usual suspects, but also distributions like Void Linux, Guix, and Alpine. These last three all have one thing in common: they use an init system other than systemd, because Flatpak doesn’t care what init system you use. It seems that for the next major version of Flatpak, however, that’s going to change: systemd will probably become a dependency for Flatpak.