By default Neutron ML2/OVN backend provides "native OVN" DHCP service and it's enough for most of the use cases.

But OVN isn't really fully fledged DHCP server, like e.g. dnsmasq and there are DHCP features which it can't provide. Because of that there are some knows use cases which may require to run neutron-dhcp-agent still. One of such use cases is support for iPXE over IPv6 for baremetal provisioning (see https://issues.redhat.com/browse/OSP-20111). This use case can be actually solved by using recent enough OVN as it is supported since some time there.

We need to make research and discover any other potential use cases in which using native OVN DHCP may not be enough and will require use of the neutron-dhcp-agent.