They also have a website with docs: https://nixmultiverse.com/docs/

The website can also be used to search for specific package versions in nixpkgs, to get the nixpkgs revision: https://nixmultiverse.com/

The above feature is similar to: https://lazamar.co.uk/nix-versions/ or https://www.nixhub.io/ , but using this project is a way easier alternative than searching for an pinning revisions.