What problem does this feature solve?

When watch listens on ref, depth listening should be performed if ref is an object type.

What does the proposed API look like?

 if (isRef(source)) {
    getter = () => source.value
    forceTrigger = isShallow(source)
    if (isObject(source.value)) {
      deep = true
    }
  }

That would be a breaking change. Also, you can do that today by setting the deep option yourself.

So this wont be implemented.