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.
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_...).