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.
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.
Does Minecraft's offline mode not work indefinitely? I'm not familiar with the official launcher but this article [1] doesn't say anything about needing to reconnect occasionally.
> First, you need to make sure Minecraft is installed on your device. Once this is complete, log in to the launcher while you are still online. This will save your login details for offline access.
A connection to Microsoft's servers will also be required to download the game in the first place, so I'm not sure this is really a problem (for compliance with the bill). I think the official launcher won't even let you download the game without authenticating first, but I'm not sure.