It seems like there is no official way to do that (see on the NixOS Discourse: Why is there only one version in nixpkgs?).

Some options that are available for that purpose are:

• https://lazamar.co.uk/nix-versions/ (can filter by channel)
• https://history.nix-packages.com (seems to be the spiritual successor of lazamar, but can't be filtered by channel)
• https://www.nixhub.io/ (is more focused on devbox and also can't be filtered by channel)