TestFlight release notes are one of those things that always end up being typed into a web form by hand, right at the end of a build, by whoever happens to be around.
This is a GitHub Action that uploads them for you. You give it your App Store Connect API credentials, the app id, the build number, the platform and the text, and it sets the "What to Test" notes on that build. It handles iOS, macOS, tvOS and visionOS.
It's Python, talking to the App Store Connect API, signing its own JWTs. You can run the script locally with a few environment variables if you want to test it outside of CI.