Nix 2.4 is totally compatible with NixOS. It was held back from default status on the latest NixOS release because it includes a few behavior changes to the CLI that have been jarring for some users. It's to ease the transition for them, not because it's broken or something like that.

You can set up Nix 2.4 on NixOS 21.11 by adding

  nix.package = pkgs.nix_2_4;

in your configuration.nix :)

I like the new CLI, too, and it's worth trying if you want to see what all is new in Nix. Here's how to enable flakes on NixOS, if you have Nix 2.4:

  nix.extraOptions = ''
    experimental-features = nix-command flakes
  '';

It has been released. The 20.11 release contains both a nix_2_3 and a nix_2_4 package. I'm fairly certain the only reason 20.11 defaults to 2.3 is because the development of 2.4 focused a lot on flakes despite the fact that there are still an 'experimental' feature (experimental like Gmail was a beta for 7+ years). If you aren't using Nix already there is no reason you can't just start using 2.4 with flakes enabled.