Why don't provide a AbortSignal on startTransition?

The official docs for startTransition bring a problematic example, which is also mentioned in FAQ.

But why doesn't the React team add an AbortSignal to the transition function? That way we can just check the cancellation status to avoid the dirty rendering.

I understand it might be useless once the AsyncContext has been delivered. But before that, it makes the official example work correctly, right?