Would that help? sshd, by design, opens shells. the backdoor payload was basically to open a shell. that is, the very thing that sshd has to do.

The pledge/unvail system is pretty great, but my understanding is that it do not do anything that the linux equivalent interfaces(seccomp i think) cannot do. It is just a simplified/saner interface to the same problem of "how can a program notify the kernel what it's scope is?" The main advantage to pledge/unveil bring to the table is that they are easy to use and cannot be turned off, optional security isn't.