As per https://github.com/NixOS/nixpkgs/tree/master/pkgs#patches:

When adding patches in this manner you should be reasonably sure that the used URL is stable. Patches referencing open pull requests will change when the PR is updated and code forges (such as GitHub) usually garbage collect commits that are no longer reachable due to rebases/amends.

In the following cases, a .patch file should be added to Nixpkgs repository, instead of retrieved:
[...]
• has a high chance to disappear in the future due to unstable or unreliable URLs

Many patches are currently using commits from opened pr and are subject to this issue. Lets fix this

nix run nixpkgs#ripgrep -- -U -n \
  'fetchpatch2?\s*\{[^}]*?\burl\s*=\s*"([^"]+)";' -r '$1' \
  pkgs | grep "github.com"