No response from GitHub yet, but the failing status continues to confuse contributors. As of gh-18417, we're temporarily reporting "success" for these cases since that's more accurate. We'll revert that change once the underlying issue with GitHub has been resolved.

A fellow GitHub user has observed that this feature may have been intentionally removed:

The neutral exit code isn't mentioned in the new documentation, so I don't think it's supported anymore.

We still haven't received word from a GitHub representative, though.

A GitHub employee has confirmed via Twitter:

We did, we'll have a different neutral exit strategy. exit 78 seems unique, but isn't. (eg, git merge exits with the number of conflicts encountered.)

It sounds like the neutral status will be supported in the future, so maybe we ought to keep this issue open until that's available.

We're no longer using the neutral code for GitHub Actions, and I doubt it's coming back to GitHub Actions. It makes more sense with the generic GitHub Checks API, where actions can be taken to resolve such inconclusive checks.