LinuxSandboxing was updated 12 May. The new version of the document adds:
“The namespace sandbox aims to replace the setuid sandbox. It has the advantage of not requiring a setuid binary. It's based on (unprivileged) user namespaces in the Linux kernel. It generally requires a kernel >= 3.10, although it may work with 3.8 if certain patches are backported.
Starting with M-43, if the kernel supports it, unprivileged namespaces are used instead of the setuid sandbox. Starting with M-44, certain processes run in their own PID namespace, which isolates them better.”
So the answer is that SUID is disabled because the new Namespace Sandbox replaces it (if possible).
