The rule was 11.13 of the Apple App Store Review Guidelines:
11.13 Apps can read or play approved content (magazines, newspapers, books, audio, music, video) that is sold outside of the app, for which Apple will not receive any portion of the revenues, provided that the same content is also offered in the app using IAP at the same price or less than it is offered outside the app. This applies to both purchased content and subscriptions.
The same-price rule was revoked in 2011 after legal challenges from newspapers & magazines. It was replaced with a rule that doesn't allow linking to any websites within the app.
https://www.macrumors.com/2011/06/09/apple-reverses-course-o...
As of 2020, rule 3.1.3(a) also does not allow you to recommend in any public business communications using payment methods other than Apple:
"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."
https://developer.apple.com/app-store/review/guidelines/