Surely if they are still the duke and duchess of Sussex though it’s not exactly any different?
It affects precedence within the Royal Family. If a HRH is removed it means you come after everyone else who has one, and have to curtsey to them etc. Commoners would not be expected to curtsey to a Duke who was not a HRH.
I remember reading that Princess Michael of Kent told Diana not to curtsey to her after her HRH was removed, even though she should have done, because she sympathised with Diana.