It is illegal in Ontario. From the Ontario Highway Traffic Act:
Number plates, further violations
No other numbers to be exposed
13 (1) No number other than that upon the number plate furnished by the Ministry shall be exposed on any part of a motor vehicle or trailer in such a position or manner as to confuse the identity of the number plate. R.S.O. 1990, c. H.8, s. 13 (1).
Number plate to be kept clean
(2) Every number plate shall be kept free from dirt and obstruction and shall be affixed so that the entire number plate, including the numbers, is plainly visible at all times, and the view of the number plate shall not be obscured or obstructed by spare tires, bumper bars, any part of the vehicle, any attachments to the vehicle or the load carried. 1994, c. 27, s. 138 (7).
Obstruction prohibited
(3) The number plates shall not be obstructed by any device that prevents the entire number plates including the numbers from being accurately photographed using an automated speed enforcement system. 2017, c. 9, s. 3.
Same
(3.0.1) The number plates shall not be obstructed by any device that prevents the entire number plates including the numbers from being accurately photographed using a red light camera system. 1998, c. 38, s. 2 (1).
Same
(3.1) The number plates shall not be obstructed by any device or material that prevents the entire number plates including the numbers from being identified by an electronic toll system. 1996, c. 1, Sched. E, s. 2 (1).
Offence
(4) Every person who contravenes subsection (2), (3), (3.0.1) or (3.1) is guilty of an offence. 1993, c. 31, s. 2 (5); 1996, c. 1, Sched. E, s. 2 (2); 1998, c. 38, s. 2 (2).
Right. You wouldn’t be able to tell by just walking around though. Every third cars has one of these plate darkener screens. And many cars also have windows so tinted you can’t even tell if it’s a man or a woman driving.
Number plates, further violations No other numbers to be exposed 13 (1) No number other than that upon the number plate furnished by the Ministry shall be exposed on any part of a motor vehicle or trailer in such a position or manner as to confuse the identity of the number plate. R.S.O. 1990, c. H.8, s. 13 (1).
Number plate to be kept clean (2) Every number plate shall be kept free from dirt and obstruction and shall be affixed so that the entire number plate, including the numbers, is plainly visible at all times, and the view of the number plate shall not be obscured or obstructed by spare tires, bumper bars, any part of the vehicle, any attachments to the vehicle or the load carried. 1994, c. 27, s. 138 (7).
Obstruction prohibited (3) The number plates shall not be obstructed by any device that prevents the entire number plates including the numbers from being accurately photographed using an automated speed enforcement system. 2017, c. 9, s. 3.
Same (3.0.1) The number plates shall not be obstructed by any device that prevents the entire number plates including the numbers from being accurately photographed using a red light camera system. 1998, c. 38, s. 2 (1).
Same (3.1) The number plates shall not be obstructed by any device or material that prevents the entire number plates including the numbers from being identified by an electronic toll system. 1996, c. 1, Sched. E, s. 2 (1).
Offence (4) Every person who contravenes subsection (2), (3), (3.0.1) or (3.1) is guilty of an offence. 1993, c. 31, s. 2 (5); 1996, c. 1, Sched. E, s. 2 (2); 1998, c. 38, s. 2 (2).