Every night the list of packages is exported and then merged into a single CSV file that has every package version.

The CSV has a column for the pname, the nix package name, the name, the version and the sha of the commit it was added.

For example, to find which sha go-15.10 appeared, you can run

curl -sSfL https://ahobson.github.io/nix-package-search/nix/nixpkgs-unstable/all_packages.csv | grep 'go-1.15.10'

The result would look like

go,go_1_15,go-1.15.10,1.15.10,674c39c08fb94d315cb9bc699da33fd1be4b4e2e

If you enable javascript, you will be able to search packages via your browser.