It's not a networkd default. Networkd doesn't kill or even care about other services. The setting exists on the service's unit side. You have to explicitly specify how you want to depend on networking.

Try it. Pull the Ethernet cable.

I do that every day moving between WiFi and plugged in Ethernet. It seems you're misunderstanding what exactly is killing processes in your case. Try to collect the logs and maybe ask on serverfault. I promise networkd is not killing your processes and if you don't have strict dependencies in your unit file, systemd is unlikely to do that either.

An ethernet interface is not a netdev. Netdevs are always virtual interfaces.

it is a good thing you have no states to care for.