Source is here, from a Unity WebGPU thread. Look at the comment from October 27 from Brendan Duncan, a Unity employee: https://discussions.unity.com/t/early-access-to-the-new-webg...

"I have found that WebGPU is enabled by default now with iOS 18.2. Apple has been working in the open on WebGPU. The WebKit source code has their latest WebGPU work in it. What hasn’t been known is their release schedule, but now with 18.2 it’s looking very promising that it will be on by default in that version."