We still use net30 as the default topology in OpenVPN server configs. However, it's deprecated (see https://community.openvpn.net/openvpn/wiki/Topology#Topologynet30) and also doesn't work properly with Windows clients, so it's a rather bad default.

We should add deprecation warnings, set the new default to the current recommended option — subnet. However, we also need to add a migration script so that people who rely on net30 do not end up with broken setups.