Is it common practice for Canadian restaurants?

Depends on what you mean by "yesterday's food" . If you are implying fast food chains keep the cooked meat for the next day, and serve them in the same format, they don't and in most cases, near the end of the day, they will make meals to order to avoid waste. If you are talking cut vegetables, condiments etc, yes they will use them the next day.

In 30 years restaurant business, I know some will keep soup over for the next day, and cooked chicken will be used for sandwiches or soup. Wendy's will purposely cook more meat than required, to be used in chili, but they will not used cooked burgers for the next day

I think it depends on the restaurant, a large chain or a smaller ma and pa place. Some places do re-use things like soup, but if its burgers, fries or any type of fried food they typically throw it out. I know that the cost of running a restaurant is extremely high versus the profit, so the smaller place may be more apt not to throw food out. I don't think there's regulations against using day old food, but there is many regulations on temperature of the food, how the food is prepared and stored.