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).
rrbender
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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).