If we don’t authenticate your account, how could we verify that you have access to a server or that you even have a Plex account. Or in your case a Plex Pass. You need an internet connection, at least for a little while to authenticate this information.
If you don’t want any type of authentication, then do not sign into your server. If your server isn’t signed into your Plex account, then you don’t need to authenticate the clients that access it.
