Originally Posted by
Johners
The key difference between the official macOS client and running the Windows client via WINE/Proton is that only one needs a third party launcher.
On Windows, the launcher uses a proprietary Microsoft standard that doesn't work in WINE/Proton so you have to setup a version of XIVLauncher in order to actually play the game. If we run that against the word-for-word terms of service, it's a third party tool and breaks the terms of service to do so. If SE ever change the launcher so it runs fine in stock WINE/Proton, there's nothing else that suggests you'd be breaking the ToS, you'd just receive no customer support as they don't officially support this use case.
On macOS, Square Enix ship the game with a CrossOver compatibility layer that works with default launcher. This doesn't break the terms of service because no additional third party tools are being used but rather it's a custom and officially supported compatibility layer.