i think that GEN 6, OEMs/Car Bands are still with the plataform rights, Ford had the Fusion on some games outside NASCAR license, Chevy also maneged to do some products with the NASCAR Camaro outside NASCAR license, such as a LEGO model
I think Forza can pull it bringing gen 6 cars back or bringing the Gen 4 cars since they are classic cars that are not used anymore and used on many categories outside NASCAR as well, the way they had put that Toyota 2JZ on FH5 may be a indication/test that they may try to do it again on FM