You have to remember, the pandemic hampered travel, otherwise I think we would have seen Magny Cours in the 2020 update and I trust that, once they are able to, Kunos will send staff to France in order to get the data they need to build Magny Cours and release into those who bought the 2020 DLC. They have done this in the past. I would think the same would go for the Mustang and Jaguar GT4 cars.I hope they don't continue releasing "half content" dlc, like the 2020 one with only Imola and 2 cars(EVO's...). If they release one for the american GT for example, it should include all tracks and not only like 3 and call it the american GT DLC.
Chances are we will never get Magny Cours or the updated Zandvoort since the dlc that would include those is already out.