Just from Google Images it seems it was removed around 1936/7, around the year of the International Exposition. Any ideas why? Presumably something boring like increasing visitor capacity for the exhibition? They were only about 50 years old when they were removed so I doubt it was structural decay? Such a shame, I think they vastly improve the towers appearance. Looking at high-res images, they don't seem too complicated to replicate, would love to see them restored someday. Maybe for their 150th anniversary, but I imagine Notre Dame is eating up any restoration money these days.