In practice over the years, it came to mean in broad popular opinion the item on the menu to be avoided, because it would be the item that was so old it had be pushed out of the kitchen or that was composed of mystery-meat that the cook didn’t want to identify. It became applied in a deprecating way to non-food items with phrases such as “toxin du jour” and “theories du jour.”
Restaurants had begun avoiding the term by the 1990s, replacing it with the less-denigrated term “Special of the Day.” At the same time, diners began noting that the “Special of the Day” had become always more expensive than any regular items on the menu.
In any event, there was never any reason to use the French “Soupe du jour.” Many translators put their heads together and came up with an alternative English term: “Soup of the day.”