Not really. It's more like the GT6 premiums and the upgraded standards, but they have actual cockpits and stuff as well. Although even that is over-selling it. Premium vs. standard was glaringly obvious even from a distance, the older cars in Forza you mostly have to go looking for the differences. Forza has a handful of cars that have significant inaccuracies that should really lead to them being rebuilt, but the number is not large. I'm not sure it's in double digits, although we'll find out when the game releases. They could do better, but it's hardly a deal breaker.
No other game has ever had a disparity of content like the GT5 standards and premiums to my knowledge, because other developers recognised that no assets are entirely future proof. Even if you keep upgrading the same base model, there's still work that gets put into them every generation that makes them distinct from what they were 15 years ago. The same is true of premiums that got built for GT5P, moved to GT5, upgraded to GTS and then migrated into GT7. GT5P came out in 2008 so those models are likely of a similar age to what you're talking about or older. But that's not seen as a problem because it isn't one. Old models are fine if they're upgraded well.
It's the only way making these massive car lists is viable in a modern game. Making 500+ cars from scratch to the level of quality these games require every 6 years isn't reasonable when there are perfectly good assets there that just need a touch up. Yes, Forza should fix the small number of cars that are inaccurate. But they're a problem because they're inaccurate, not because they're old.