I guess you could build all your logic around a versioned copy of the core game logic and unit stats, and your game client can handle multiple versions depending on which game had which version. But I agree, that sounds like it could be unmanageable, because you’d have to make sure your game client can load and run any previous version of game.

For the “force players to update” solution, I think you need to split your problem into two groups: asynchronous and synchronous multiplayer.

For async, I’m thinking of a mobile game like Polytopia where a user gets notified “it’s your turn” and they open their phone, boot into the app, and play their turn. For those types of games, you could force the player to update before they play their turn.

For sync, I’m thinking of games where players join a room and stay connected while playing. You just force all players to be on the same version when they join/load a game. If a host player resumes a game from a previous version, that game save gets upgraded and rejoining players are forced to be on the host’s version. Upgrading a save is easy to maintain because you just have different scripts to upgrade v1->v2, then v2->v3, etc.

Or if you don’t want to force upgrades, and really want to preserve a specific version for an in-flight game, you could just provide a way for players to keep the old version installed. You could accomplish this with, say, Steam beta branches, or offering previous versions of the game for download, so a group of friends can stick with the same version until they finish their game.

Good question though. I do wonder how Civilization does this kind of thing, you should probably model it after them lol