Snapd is actually part of Debian 9 (Stretch), which means that Snap packages are also supported on Debian, as well as Ubuntu, Fedora, SUSE, CentOS, Gentoo, Yoct

Snapd is actually part of Debian 9 (Stretch), which means that Snap packages are also supported on Debian, as well as Ubuntu, Fedora, SUSE, CentOS, Gentoo, Yocto, Arch, etc. Flatpak exists for one and only one reason -- because Red Hat, Inc. refuses to work with technology of any kind led by Canonical, period.