>>396181108
The level of foveated rendering can be changed to give a good balance of performance to visual quality. Even when lens-matched, it's still better than not having any at all. 
As for Quest foveated rendering, iirc there was a bug that made edges unaligned which made it more noticeable than it should've been. 
Back in the day, in fact, Nvidia did have other methods to do fixed foveated rendering, and games like Raw Data and that Batman game actually used it. 

>>396181181
How does that work? Vulkan is also simply just an API. 
Varjo was only able to do FR, according to their documentation, by either using their extension so that you render 4 views, or by using VRS. 
Nvidia themselves state that foveated rendering is only possible because they modified their hardware itself to do it. Otherwise you run into trade-offs like what you get with the quad view trick Varjo did unless used in tandem with SMP.