There is no expectation of any channels, though instead it assumes that the path <nixpkgs> resolves. Read more about the resolution process in man nix, somewhere near the -I and NIX_PATH.
You might be able to use nix-shell -p '"nixos-20.09".hello' in your current situation.
Though the better way is probably to “rename” your current nixos-20.09 channel to nixpkgs as briefly described in How to switch to a stable channel and install using nix-env without needing the -f option? - #3 by NobbZ.