The spec defines the render state "composition disabled" flag in https://immersive-web.github.io/webxr/#xrrenderstate-composition-disabled.

From the implementation POV using negative names for booleans is not usually a good idea because they are less readable in general. The mental cost of reviewing/debugging code is higher as it forces web engine hackers to deal with double negations. Obviously that isn't rocket science but it's really error prone when you're debugging complex issues and it's pretty easy to for example assume that composition is disabled when the flag is set to false.