Is playing FFXIV on Linux against ToS?

[Phidaissi]

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.

This is not true, the use of that third party launcher is absolutely not required to play on Linux as the newer launcher is not actually necessary.

I'm doing it right now. It runs in Proton/Wine and there are working scripts to do it for you published on Lutris, you can look at them and see that the one labelled as using the official version only installs from an official source.