[aero_code]

In addition to the sibling comment, even Minecraft server has an "online-mode" setting, and if it's set to false, it doesn't authenticate players. Whenever my family has played on LAN, we've never used any auth servers (which is clear because we don't give Minecraft any credentials). The setting only applies to LAN players, but that should be easy enough to work around with a VPN or something.

[Akronymus]

Doesnt the client need an initial authentication to microsoft servers?

[7jjjjjjj]

You need to authenticate to download the game, but then you can play it as much as you want even offline. This is true for Prism Launcher, anyway. I haven't used the official launcher in years.