Not a grey area in the UK, albeit a frequently misunderstood one.
If a motor vehicle has a windscreen, you have to look through, it needs wipers AND a washer. Aeroscreens and deflectors are deemed something one can look over, and thus not need wipers and washers.
If you changed the screen for a windscreen, you'd need wipers and washers, but it would be a construction and use matter, strictly an MOT test fail if you hadn't, but contruction and use violations one can get away with for years, then VOSA might throw the book at you!
