Apple revised its App Store Review guidelines ahead of the iOS 14 launch, with a number of new changes including explicitly permitting game streaming services such as Google Stadia, Microsoft's xCloud and Facebook Gaming.
That's a change from previous policies, but there are conditions: Apple says the games must be individually listed on the App Store and downloaded via the App Store, not from within a game streaming app. Game updates must also be submitted for review, and must support in-app purchases.