So, first off, even when this is allowed, the UI experience sucks, as you aren't allowed to help the user make the purchase from the app: the user has to go hunting on Google for the idea they can make an external purchase.
> Apps and their metadata may not include buttons, external links, or other calls to action that direct customers to purchasing mechanisms other than in-app purchase.
But like, while you might think it is allowed--due to the existence of some apps like Netflix and Udemy--those apps are in explicitly exempted content-catalog oriented product categories, such as movies and online education.
> Apps may allow a user to access previously purchased content or content subscriptions (specifically: magazines, newspapers, books, audio, music, video, access to professional databases, VoIP, cloud storage, and approved services such as classroom management apps), provided that you agree not to directly or indirectly target iOS users to use a purchasing method other than in-app purchase, and your general communications about other purchasing methods are not designed to discourage use of in-app purchase.