I'd like to make a general request to do some research and see if there's a specific use case that drove the OpenXR design of the same feature, which allows depth sorting to be toggled per-layer, but otherwise this seems OK with one naming nit.

I asked internally and that flag is so the system can make sure that quad/cylinder layers can always be drawn on top of the scene.

For instance, if you hit the Oculus button during an immersive session, the dialog that then pops up should always take precedence over what the browser is drawing. Same with the keyboard which should always be in front.
I don't think that's a particular concern for WebXR.

Agreed. Thanks for looking into it!

Looks like that in Oculus2 there are a flag, but still not working.
I'm tried to use it - session starts, but quad layers not sorted between themselfs, need to sort manually by Z across camera.

Yes, there is a flag but the feature is not enabled yet. The flag will just trigger all the logic in the browser but the OS won't honor it.
We'll announce when it's ready for experimentation.