That actually is illegal. I expect police to start caring more about this as they lean more heavily on ALPR.
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).