It's a very restrictive subsidy. The details (in French): https://www.service-public.fr/particuliers/actualites/A15906
You need to trade-in a car or a van you own since at least a year and it needs to be from before 2006 or 2011 if it's a diesel. Then 40% cap is 1500€ for everyone and if you are way below minimum wage it's up to 3000€. An additional 1000€ might be available depending where you live (low emission zone which include at the moment 10 of the biggest cities and more to come).
That restriction means you cannot drive the vehicle in those zones during some periods or when there is a pollution event.