PSA for Civilization V aficionados: the Windows and Mac versions are no longer compatible for online multiplayer.
It seems the game’s online state management is probably based on passing raw game structures, and some types differ between 32-bit and 64-bit versions: the Windows version of the game is 32-bit, but the Mac version was updated to 64-bit last year to allow it to run on recent versions of macOS that dropped 32-bit support.
It’s unclear whether updating the Windows version to 64 bit would resolve the incompatibility, as macOS and Windows have some different types at 64-bit as well.
This was an avoidable problem, by either using device independent serializations or device independent core representations. And it was exacerbated by Apple dropping 32-bit compatibility and forcing developers to make rushed decisions about supporting or abandoning legacy products.