Comment 1 for bug 1872589

upstream systemd has actually changed the behavior of UseRoutes; now, it only disables actual defined routes provided by dhcp, and does not control the gateway route. There is a new option, UseGateway, that controls if the dhcp-provided gateway route should be used or not.

More details in bug 1867375 as well as references to the upstream changes. As noted in my comment there, if UseGateway is backported to Bionic, its default needs adjustment to default to the provided value of UseRoutes; maybe that should be done for Focal as well.