The sticking point here seems to be the DCO, which the “signed off by” thing attests. With that attestation other people who sign-off to the DCO can carry the p

The sticking point here seems to be the DCO, which the “signed off by” thing attests. With that attestation other people who sign-off to the DCO can carry the patch forward (maximum to a pull request against Linus' tree). Without it though you have a broken chain. But it ain't much of a sticking point. Most of the time it is just an oversight that can be pointed out and corrected. All it takes is for the maintainer to point it out and ask for it and then then the author to do it. (The patch author themselves need to do it or else there wouldn't be a point to the sign-off; forging a sign-off would defeat the purpose.)