Not required, nixos-rebuild-ng works fine without it. It is just showing you a warning because some functionality will be missing, but unless you depend in the .version-suffix file being generated it should be harmless.
BTW, nixos-rebuild did the same thing, not sure why it didn’t trigger in your specific configuration though:
Edit: I think I found the issue, I will open a PR soon to fix it.
This was an old note before nixos-rebuild-ng was declared stable and I forgot to remove. Pretty much everything in that README.md is out of date. nixos-rebuild: remove by thiagokokada · Pull Request #466809 · NixOS/nixpkgs · GitHub updates the README.md removing out-of-date information.