I don’t know what it’s for, but I believe you get that if you use nixpkgs.url = "https://channels.nixos.org/nixos-YY.MM/nixexprs.tar.xz"; instead of nixpkgs.url = "github:nixos/nixpkgs/nixos-YY.MM", see discussion here: Questions about `inputs.nixpkgs`.
I suppose I might as well also post a link to my servers’ configuration git tree: https://git.sr.ht/~zackw/server-configs/tree/nix-and-guix/item/nix The machine we’re talking about is “tinka”.
Incidentally, the only thing I’m using flakes for is to pull in sops-nix and disko. Experimenting with npins or niv instead is on The List but it’s fairly low priority.