I would also love to see that dhclient issue resolved upstream, or at least a cleaner way to work around it. But we should also be mindful that for most workloads the impact is probably way, way less.
Some of these things really only show up when you push things to their extremes, so it probably just wasn't on the developer's radar before.
I believe systemd-networkd has its own implementation of DHCP and therefore doesn't use dhclient. But I wonder if it's behavior is any better in this respect.
systemd-networkd keeps open that kind of socket for LLDP but apparently not for the DHCP client code. wpa_supplicant also keeps open this type of socket on my local system. and the dhcpd daemons on my routers have some of those too for each interface...
i wonder if the slow path here could be avoided by using separate network namespaces in a way these sockets don't even get to see the packets...
Some of these things really only show up when you push things to their extremes, so it probably just wasn't on the developer's radar before.