Syscall origin verification is orthogonal to pledge and is not really even a security feature at all without strong CFI. The general consensus right now is that strong CFI is kind of a mythical unicorn that doesn't exist. pledge remains useful even though this is true, so syscall origin verification is not a prerequisite.
Yeah I said "compare" just as an illustration of the control that OpenBSD can and does exercise over the interface between user space and kernel space.
