There is already a convention for signed-off by. I don’t understand either why GitHub is pushing for this, though I have seen this feature before on some repositories before the post.
Edit: Now that I think about it, can see why. I have seen merges where is was good enough to merge, but needed som slight adjustments. Rebase/merge with minor changes would be a good use for this, and would allow for better crediting of both authors.
It's not clear to me what the semantics of Signed-off-by is supposed to be, but at least for some projects (e.g. the Linux kernel), it doesn't imply that you are a co-author.
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.
Edit: Now that I think about it, can see why. I have seen merges where is was good enough to merge, but needed som slight adjustments. Rebase/merge with minor changes would be a good use for this, and would allow for better crediting of both authors.