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.
Some other comments in this thread (i have no idea how true they are) suggested there were serious backwards-incompatibilities preventing nix2.4 from being default in the last NixOS release.
