The policy is around but not always enforced, William and Kate seemed to have followed it mostly but today Princess Eugenie was there with her longtime boyfriend. I think her not being at the ceremony ( it is said she will be at the reception) comes from the fact that when the majority of this wedding was being planned (invitations sent out) no one anticipated how serious the relationship would become. It would of been odd to give Harry a plus one to someone they weren't sure was even going to still be around. Also the ceremony was said to be smaller then rest of the events for the day (100-150 at the ceremony, 300 at reception).