Comment by adrian_b

Unfortunately, there is no need to advertise separately in CPUID that a CPU is backdoor-capable, because other features implicitly specify whether the processor supports backdoors.

Since Intel has added the System Management Mode in 80386SL (October 1990), all Intel and AMD CPUs that support SMM implicitly enable the insertion of backdoors in the BIOS/UEFI firmware. ARM has followed the Intel example by introducing EL3 (Exception Level 3), which is equivalent with Intel SMM and it also enables the insertion of backdoors in the computer firmware.

Most modern CPU vendors have not remained content with enabling firmware backdoors, but they have added separate management processors, which may be shamelessly named "security processors", enabling backdoors that are even harder to detect and disable.