From a bit of googling, it seems you stumbled across a known backwards compatibility breakage. There was an attempted fix in Be mindful about the default arguments passed to nixpkgs in nix-shell by andir · Pull Request #5543 · NixOS/nix · GitHub but that was never completed.
Also from that thread, the only way to fix it is to use a version of Nix <= 2.3. Based on the specifics of the problem I’d guess you might be able to install it declaratively in your config files, but I haven’t tested it.
I suggest you report that problem to the creator of that search tool since every nix-shell invocation they give for nixpkgs older than 19.09 will fail with any Nix > 2.3. I don’t think there exists official Nix documentation with those instructions otherwise we’d need to update it there as well.