Is it legal to keep your trailer hitch attached to your truck when not pulling a trailer in Ontario Canada?

No it is not legal anywhere in Canada. It poses a threat to pedestrians, as well as drivers not only on the road, but in parking lots as well. They stick out too far and are too blandly colored to be seen to be left on when not towing.

You can thank the mailmen of Canada for making this illegal-as they seem to bust their knees a lot on them and filed complaints.