NixOS and GuixSD are distros built entirely around Nix and Guix, respectively.

In a word: yes! That I know of, Guix, Nix, and Luet are the only ones with whole, bootable Linux distros built on top of them so that the 'package manager' also does configuration management in some form. (IIRC, Luet is more traditional here, it manages config files only state fully and partially, using hooks and triggers.) Those distros are very worth checking out! The operating systems built on them are GuixSD, NixOS, and MoccacinoOS, respectively.

Distri is a system-wide package manager, but its distro is a PoC. It has a small package collection and no users, last I checked. But it has some cool innovations and it's very fast, so if you want to play with building a distro on a next-gen package manager, it seems great!

I think Denxi could be used to build an OS in a similar way as Nix, but I'm not aware of any such effort currently. The same is true of Spack.

Haiku and RedoxOS have some fancy Nix-like package management features as well, in terms of rollbacks and multiple versions of transitive dependencies playing nice together on the same system, and maybe in some other ways, too.