I mean... NVidia has their own fork of ReShade included in GeForce Experience (under the name "Freestyle"), and FFXIV is one of the
officially-supported games. (Scroll down to spot it, the list of games is Very Long.) You can even import additional/custom ReShade shaders into Freestyle by dropping them in the appropriate directory.
So if you're running GeForce Experience and have the feature turned on, congrats, you're *already* running ReShade; you could hit the hotkey and bring up the Freestyle interface in-game *right now*.
Anyway, if ReShade variants were expressly forbidden, I suspect Square-Enix would not have allowed FFXIV to be one of the officially-supported games for Freestyle.