I'd say adding disused is definitely one solution. Given these bus stops were originally imported 14 years ago (https://www.openstreetmap.org/changeset/3740505) there have been and will continue to be changes, so it also seems OK to delete, especially if the sign is gone and route changed. When the sign is still there, but another sign added that the bus no longer stops there, disused probably makes the most sense. Happy mapping!