You don't need the server to be open source. You just need to be able to run it yourself without any central server involved.
This has been very much the norm from the nineties up until fairly recently. I spent my childhood and teens playing games over LAN or the Internet with self-hosted servers (Doom, Duke Nukem 3D, Half-Life, GTA, Dungeon Siege, Red Alert, Warcraft 2, ...)
This has been very much the norm from the nineties up until fairly recently. I spent my childhood and teens playing games over LAN or the Internet with self-hosted servers (Doom, Duke Nukem 3D, Half-Life, GTA, Dungeon Siege, Red Alert, Warcraft 2, ...)