The trap receiver does accept traps from unmanaged nodes -- the nodeid is set to 0 (zero)
That is correct. This feature was introduced in 10.0. There is no present way to enable the previous behavior.
There is a difference between nodes that are not managed by Orion and those that are but are in an unmanaged status.
Nodes managed by Orion but in the unmanaged status will throw away their traps (and syslogs). In the traps service log file you would see a warning level message like:
Trap has been received from device that is unmanaged in Orion. The trap message has been ignored.
There is currently no ability to override this behavior.