While I think it's quite clearly not a case of "85 today is a 95 of early last gen"... I do think there are plenty of cases where a game early last gen could score within the 95 region, but would score in the realm of 85 if released in exactly the same form (except with modern graphics) today. This makes quite a bit of sense if we assume that newer games build on the knowledge of those that came before, otherwise you'd need to go above 100 in short order.
Forza Motorsport 6 (since you use Forza as an example) has 86 on metacritic, and it honestly beats the breaks off Forza Motorsport 3. The difference is that it doesn't beat other games relative to its time in the same manner as FM3 did in 2009. Forza Horizon 3 despite sharing the Forza brand is a completely different matter and makes nearly anything you directly compare it to look pretty bad, which is why it scores so highly. I haven't played UC4 (not much of an Uncharted fan), but I'd guess its the same with that too, in order to warrant its scores.
It's definitely true that reviewers certainly aren't handing out crazy high scores like they were last gen though. Back then almost anything with a budget was a lock for 90+. The current review curve is far better imo.
