We knew going from 900 at the end of FH5's life to 550+ in FH6 was going to mean there would be some unfortunate cars not ported from FH5. And especially since all the cars need QOL updates - New Forza Aero (as much as it is a disappointment IMO), Paintable window stickers, different rims front to back. And that's not including physics, QA testing, licensing, the 10 or so remodeled cars etc.
Still, we get two Vulcans and not one modern Jag? Not one F-Type?
We get 3 Modern Dodge Challengers but no 70's road going Escort?
We get 5 different Audi R8's - but we are still stuck with the 2013 Merc A-Class. And a 2010 Audi TT, when last game we had 2018 and 2020 models.
We could go on and on.... However IMO the Car list still looks really good despite the interesting omissions.. For all that we've lost, we do have the strongest line ups from Nissan, Honda, Mitsubishi, Subaru, Toyota and Mazda we've ever had in Horizon. And for a game set in Japan, that matters the most.