Foveated rendering is supported by default. Most apps will rely on the OS's renderer and not their own so they don't have to do anything with eye tracking to have it enabled.
EDIT: See cma's comment below which adds more info: apps that use their own render still support foveated rendering too.
If they try to hide it by restricting access to reading the foveation texture, you can be sure anti privacy apps like Immersive Facebook will try tricks like recovering it from GPU timestamp queries (render two transparent triangles covering the screen in separate draw calls, find which took the longest, subdivide and repeat).
Exactly. If you want to do anything without going through Apple's benediction, you're shit out of luck. It's one more episode in the war on general computing, except somehow it's okay when Apple restricts what you can do with your three thousand five hundred dollar ski goggles.
To me it seems like less of a war on general computing and more like Apple is hoping to prevent mixed reality or "spatial computing" from becoming an extension on the privacy disaster that smartphones became.
This seems prudent, because as personal as smartphones are, a headset like the Vision Pro dials that up tenfold. Considering what third parties have done with just the information surfaced in mobile operating systems, or heck even the web, I shudder to think of what they'd do with gaze data and a high fidelity color 3D map of your surroundings.
If there's a way to accomplish this without opening headset users into being easily socially engineered into figuratively selling the farm, sure.
In this particular case I think perhaps a better approach would be to allow apps to bypass App Store restrictions so long as their source code is public and binaries match that code. This would naturally deter those with shady intentions, allow FOSS projects to thrive, allow both manual and automated third party vetting of apps, and help users better know what they're getting into.
Yeah. To state the obvious: Apps will no longer run full screen; they will run as a 2D window in a 3D space. When Apple allows it the user will be able to ask the app to go full 3D (well, hopefully).
All of this is: Apple taking further control of the experience.
EDIT: See cma's comment below which adds more info: apps that use their own render still support foveated rendering too.