From Ontario Rules and Regulations guess that answers that
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 a photo-radar system. 1993, c. 31, s. 2 (5).
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).
Note: On November 20, 2004, unless the Lieutenant Governor issues a proclamation before that day, subsection (3.0.1) is repealed by the Statutes of Ontario, 1998, chapter 38, subsection 2 (3). See: 1998, c. 38, ss. 2 (3), 7, 8.
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).