highway=trunk and highway=trunk_link have the same status, the latter represents slip roads. And yes, all slip roads should be highway=x_link, it's just that in some special cases, like the southern junction of the A38 and A516 here, there's no definite point where slip road turns into main road, and so the whole slip road is marked as trunk. In this northern junction, there is a definite end point - the roundabout. Hence these should be marked as trunk_link. The ref or route of any road is irrelevant, as per my example of the A515/A50 multiplex.