> […] the game either halts completely or forks so that they can't rejoin.

When a player looses the connection the game just continues. Usually one of the remaining player then pauses the game until the player who lost the connection returned.

The game state becoming out-of-sync is often a problem of players using buggy mods. That this happens without mods is pretty rare and of course clearly a bug.

> It's also impossible to save and restore in multiplayer.

It seems you haven't played 0ad in a while, as that's possible since a27 (see https://play0ad.com/new-release-0-a-d-alpha-27-agni/).