The current rules imposed by Apple prevent game streaming services from publishing their applications on the App Store.
In practice, store rules prevent an application from serving as a third-party game repository. In fact, no service such as GeForce Now, xCloud, PS Now or Google Stadia can publish an application for iOS.
Online game streaming services work by hosting games on servers in the cloud, essentially broadcasting a rendered video to user devices. This means that a title can be played on a mobile device, but with the graphic fidelity of a high-end console or PC. The trade-off is the latency of the streaming, although companies are struggling to overcome these issues.
However, none of these services will reach the App Store, at least until Apple changes the few rules. A somewhat distant choice, given that the company recently launched the Apple Arcade subscription service. Of course, Apple Arcade works differently from game streaming services. The service only offers games available by subscription. In short, Apple Arcade is not a streaming service, as the games are downloaded and saved locally.
Unsurprisingly, Apple Arcade-like services like GameClub are normally available on the App Store, despite some difficulties with final approvals.