Man pages matching name coap_register_event_handler in section 3:

coap_register_event_handler.3.en	debian 10 / libcoap2
coap_register_event_handler.3.en	debian 11 / libcoap2
coap_register_event_handler.3.en	debian 12 / libcoap3
coap_register_event_handler.3.en	debian 13 / libcoap3t64