I just installed iOS 15 an hour ago. So I checked to see if the new Apple Maps' ratings was in there. There is now a "Thumbs Up/Thumbs Down" button that allows you approve or disapprove of a restaurants Food & Drink, Customer Service, Atmosphere, and Overall experience. It also uses your on device location data to suggest photos that you took while at the restaurant, to include in your review. So it looks like it's finally live! (At least for some people)

Interestingly enough, it still has Yelp reviews beneath it. Hopefully that just gets removed completely once Apple has collected enough of their own reviews.