Man pages matching name zmq_msg_set_routing_id in section 3:

zmq_msg_set_routing_id.3.en	debian 10 / libzmq3-dev
zmq_msg_set_routing_id.3.en	debian 11 / libzmq3-dev
zmq_msg_set_routing_id.3.en	debian 12 / libzmq3-dev