[css-scoping] Allow sibling combinators for ::slotted elements

[GeorgeTailor]

I am not sure if it's a feature request or a bug as spec does not cover this case.
Anyway, the proposal is to allow sibling combinators (+, ~) for ::slotted elements, for example:

::slotted(input[type="checkbox"]:disabled) + ::slotted(label) {
  cursor: not-allowed;
}

I know that child and descendant combinators are not allowed due to performance reasons, but sibling combinator does not seem that it would introduce a lot of performance overhead.
Since we already can apply style to all slotted elements, why not allow sibling combinators as well?