Maybe not on the coldest days, but I was in Europe in early spring when it was definitely chilly ... the patios there are open and well used, with the restaurants providing blankets. People just bundle up and enjoy the patios! (The prevalence of smoking and vaping there may be a contributing factor.)
This feels like a chicken-and-egg situation. I've seen winter patios in Vienna, Stockholm, and Copenhagen just before Christmas. Some have blankets draped on chairs for people to use, and might feature additional heating. In Toronto, some patios use heaters and plastic sheeting. I think it's bizarre that as a country known for their winters and winter activities, it's assumed we won't tolerate sitting outdoors to eat. Last year Toronto's avg temp in Dec was 3C, the avg temp in Dec over the last 5 years was 1C. The popularity of the Christmas market in the Distillery District shows there's an appetite for things to do in the winter. I think if some restaurants started doing this in tourist heavy parts of the city, like on Front to capture people coming in for Leafs games, their success would catch on and spread.
Copenhagen, 6C. So warm at the tables people took off their coats.
Copenhagen, -3C
Bloor near Ossington, Feb 2021 from Google Maps. We've done it here, albeit haphazardly.
