NixOS uses systemd, although wrapped in their own DSL, example:

https://github.com/NixOS/nixpkgs/blob/24903b5de2e07cc7b65eae...

It currently uses systemd quite heavily. There at least was some conversation about making it more service manager agnostic so that others could be used, but I'm not aware of that having gotten anywhere. I haven't checked in a while though.

NixOS uses systemd.