https://bugs.winehq.org/show_bug.cgi?id=48006
--- Comment #24 from Sophira wine-bugs@theblob.org --- I've been experimenting and have found that the new launcher works when using Proton 7.0.5 (to an extent; it doesn't show the Lodestone banner pictures but seems to work for logging into the game). Also, my partner (who uses a Steam Deck) says that they're able to run the Square Enix store version of FFXIV using Proton Experimental through Steam.
I'll see if I can work on bisecting Proton versions to find the commit that got it to start working on Proton, to find what the fix was. Would I be correct in thinking that the licensing would allow the fix to be ported into Wine?