Backport pull requests that are clean backports can be integrated without reviewers (configurable per repository). This means that not all backport commit messages will have a "Reviewed-by" line.
Backport Commit Command
The /backport commit command can be used to quickly create a backport pull request for a given commit. Just navigate to the comment in source code hosting provider's web UI and add a comment consisting of "/backport <repo>". If the commit does not apply clean on the target repository then a message will be shown for the files with conflicts.