If you had to guess, what do you think the odds are of Apple supporting WebXR in the next few months? Asking for a friend.

Interesting months coming up, indeed.

If the headset rumors are true, it would be weird if Apple didn't announce stable WebXR support at WWDC. They might be focused more on native, but good support for the Web would definitely help drive adoption for the "new" platform.

Not sure why you’re being downvoted. Not being able to deploy to the most powerful devices is a real constraint of actually using the API

What bottlenecks have you come across that would solved if it ran on an iPhone?

A few years ago I helped build a WebXR (Well, I think it was the WebVR specification) site for both Android and iOS, back when Motion & Orientation Access was possible on the latter platform. It worked pretty well, until it was no longer possible to access that API and we had to spend countless hours on a static fallback for iOS. The gyroscope is all you need for a relatively novel experience, ThreeJS had a VR shooter example you could easily extend, and not needing to download an app was great for getting more people to check it out.

Why would you want this on a phone though? It's for VR headsets.

You want one API that will work on all devices. Take this demo:

https://aframe.io/examples/showcase/sky/

On a VR headset you can just crane your neck and look around. On a phone or tablet it will use the accelerometer and let you use the device as a window on the virtual reality. On desktop you can use the mouse to scroll around.