We ended up using https://itunes.apple.com/lookup?id=app_id, as found through other web searching. It returns an "average user rating" and user rating count" (number of reviews).

We are also looking for the same thing, but unable to find it. When viewing the app on App Store Connect, it uses this URL to retrieve that data: https://appstoreconnect.apple.com/WebObjects/iTunesConnect.woa/ra/apps/{app_id}/platforms/ios/reviews/summary?storefront=US Unfortunately, there doesn't seem to be an equivalent one in the published API (as of 2023-08).