If a rigidbody object falls off the world and "Object Behavior At Respawn Height" is set to "Respawn" in the VRC_Scene Descriptor, it will not respawn unless it has a VRC_Object Sync component on it. In other words, unsynced physics objects do not respawn if they fall off the world. This affects things like Local Pickups that shouldn't be synced but can still fall off the world and be lost forever.
It should be noted that setting "Object Behavior At Respawn Height" to "Destroy" will still destroy an unsynced object, it is only respawning that doesn't work.