Send back ICMP Destination Unreachable when NAT can not be performed, RFC 5508 Sec.6

[EHfive]

Rejection of Outbound Flows Disallowed by NAT, https://datatracker.ietf.org/doc/html/rfc5508#section-6

This would further increase the complexity of the BPF program and is not very useful so long as the NAT has enough ports, and generally client can handles packet dropping. So it would be on low priority.