WebGPU works just fine in Safari on my iPhone. It was enabled by default starting with iOS 18.2.
It’s disabled still on my iPhone on iOS 18.4.1. Either it was enabled specifically on 18.2, and then disabled, or you enabled it manually. (Or some other weird thing, like it’s only enabled on iPhone Pros.)
https://caniuse.com/webgpu
Caniuse says it's still behind a feature flag, are you sure you didn't enable that at some point?