Thank you. Indeed, LuCI only clears depending entries from /etc/config/network, /etc/config/firewall and /etc/config/wireless on deleting an interface. The dhcp config is not covered. Will add cleanup for it.
Are there any other artifacts not covered? Right now only sections of type alias, route and route6 are removed in the network config if the corresponding interface is deleted, I suppose it needs to be extended to rule and rule6 as well.