That would make all existing trailers non compliant and illegal without a plate. In the UK we usually have a rule that ensures they don’t (generally) apply rules retrospectively otherwise everything would be off the road everytime there was a rule/law change….eg seat belts in back of car. Even in the eu with the RCD directive on boats they have a cut off year (1996). So how it would be enforced in the near future I think would be very difficult.