[WorldMaker]

That's what I've seen. Some projects, including the Linux kernel, use Signed-off-by for Developer Certificate of Origin legal processes.

https://elinux.org/Developer_Certificate_Of_Origin

It makes sense that GitHub would use something new to avoid stepping on toes, and GitHub's mirrors Signed-off-by in implementation as a git commit message trailer.

[brianmario]

The Co-authored-by trailer isn't actually new. In fact it was already being used by over ~1M commits on GitHub before we launched the feature! :)

The git-core commit message conventions docs mention it (https://git.wiki.kernel.org/index.php/CommitMessageConventio...) as do the OpenStack commit message convention docs (https://wiki.openstack.org/wiki/GitCommitMessages#Including_...).