With a repo set up in way that git remote prune --dry-run originreturns the following:

URL: .... my repo utl...
 * [would prune] origin/some-branch
 * [would prune] origin/some-more-branches
 * [would prune] origin/main
 refs/remotes/origin/HEAD will become dangling!

stale_refs throws ValueError: Could not parse git-remote prune result: ' refs/remotes/origin/HEAD will become dangling!'
I suppose this warning message should just silently be skipped at https://github.com/gitpython-developers/GitPython/blob/main/git/remote.py#L614