I’m surprised there’s such a big turnover for packages. Looks like the total number of packages is on the order of 100k, so 20k updated packages sounds about right to me but 20k added/removed seems high, especially since nixpkgs was already pretty much the most complete set of packages out there. I guess there were a lot renames that their statistics did not catch? The last two releases had around 7–8k new packages.
I’m a fan of looking at the new modules during releases, since I frequently find cool projects that I haven’t been aware of, but that are mature enough to warrant a NixOS module: