Is it? I was looking a few months ago at Nix support in Fedora Atomic (which Bazzite is based on), and it required some minor hacks to get the /nix folder and Nix itself working properly with rpm-ostree distros, as root is otherwise immutable. Plus, Nix isn’t available by default in the Fedora repos it seems. I believe it required something along the lines of making a /var/nix folder and symlinking it, but I believe you’d also have to work around SELinux contexts on the folder and symlink. I’ve heard of people even having issues after that, so I wouldn’t consider it “official” support. Here’s an open issue thread about it