If you create/edit a new VPN connection in nm-applet's "Edit Connections", there is the "IPv4 Settings" tab, at the bottom of which you see a "Routes..." button.
Here, you just enter your subnets, the prefix length and 0.0.0.0 as Gateway to that subnet.
The option has not been removed, it's called differently and it's configured in a different place.
Lacking an OpenVPN peer i could check against, I cannot actually confirm that this works for the OpenVPN plugin, but for the VPNC plugin, it works exactly like that.
If you create/edit a new VPN connection in nm-applet's "Edit Connections", there is the "IPv4 Settings" tab, at the bottom of which you see a "Routes..." button.
Here, you just enter your subnets, the prefix length and 0.0.0.0 as Gateway to that subnet.
The option has not been removed, it's called differently and it's configured in a different place.
Lacking an OpenVPN peer i could check against, I cannot actually confirm that this works for the OpenVPN plugin, but for the VPNC plugin, it works exactly like that.