I have been developing a multiplayer game designed to run only on a local network in a computer lab (in other words, not meant to be played over the Internet). It works fine under the old Unity networking protocol and Unity Master Server, which I can use for free. Does anyone know if the new 5.1 networking protocol can still be used for free on local (non-Internet) networks? Or will I be forced to subscribe to the new Unity Multiplayer Services?
It will be free to use for everything you want to use it for. There is no exception.
Unity Multiplayer Services is there to give you a paid service for things you don’t want to do by yourself. Examples are a matchmaking and relay servers.
So for local networking you wouldn’t need the Unity Multiplyer Services. If you need a relay server then you can choose: Either you use Unity’s service or you code and host it yourself.