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.