You certainly can run distros without systemd [0] or something very different like *BSD or Mirage [2].
[0] https://forum.qubes-os.org/t/devuan-or-other-non-systemd-tem...
https://forum.qubes-os.org/t/alpine-linux-template-non-offic...
[1] https://forum.qubes-os.org/t/openbsd-as-a-ubes-os-component/...
[2] https://forum.qubes-os.org/t/mirage-firewall-0-9-0-released/...
> You certainly can run distros without systemd
Does it then become not a full OS anymore? Mirage is what I linked to above.