So now static IP addresses and routes are configured in the dhcp client daemon settings. Makes sense.

Interface settings go in /etc/dhcpcd.conf, and it looks like:

interface eth0
static ip_address=10.0.0.1/24
static routers=10.0.0.254
static domain_name_servers=10.0.0.20

The static routes go in /lib/dhcpcd/dhcpcd-hooks/40-route

It looks like:

ip route add 192.168.100.0/24 via 192.168.0.2