Apparently WebXR is gonna be available on the visionOS Webkit: https://developer.apple.com/videos/play/wwdc2023/10279 So I guess we are still many to wonder if WebXR experimental features will ever be testable on Safari for iOS?
WebXR on visionOS but not on Safari?
VisionOS comes with a Safari that supports WebXR. You can test in on the Vision Pro Simulator. Sadly, as you're figuring out, a version of Safari with WebXR support will only be available for Vision Pro and not iOS devices. This is discussed in greater length here: https://www.lowpass.cc/p/apple-vision-pro-webxr-standard-ios.