Apple added a flag. They didn't add an implementation. There is zero code in WebKit to support WebGL2. All there is is a stub that returns a WebGL1 context when WebGL2 is asked for. Feel free to dig through the WebKit code. You'll see there is no WebGL2 code, all the functions are actually missing and no code has been added in > 3yrs. Try running the tests. (https://www.khronos.org/registry/webgl/sdk/tests/webgl-confo...), click off "all" then search for "conformance2", check the box and run. You'll see almost nothing passes.
It's supported in safari, currently gated behind a setting though.