The following would be useful to have.

Repository.generate_release_notes(
  tag: str, 
  previous_tag_name: Opt[str],
  target_commitish: Opt[str] = NotSet, 
  configuration_file_path: Opt[str] = NotSet
) -> SomeGitObject

calling
POST /repos/{owner}/{repo}/releases/generate-notes

Generating Release notes without generating a release is useful for pushing them to other locations. You might want to trigger a Github Action off the creation of a Pre-Release, and then generate the notes to hydrate the release. Or you might have 3 releases, delete the second one, and then have to rehydrate the third one to include all the information, etc.