The IPv4 and IPv6 DHCP relay service implementation supports specifying a listen port to listen for DHCP packets which should be relayed. At the time of this writing only the Linux DHCP client supports requesting an address over a non standard port (UDP 67). Supporting DHCP ports is only useful for debugging but I still don't get the real use-case here.
To get a clean CLI we should remove this option. I have also not found a similar option in Cisco IOS. A migration script is required for this removal.