I just read about Apple's App Store Review Guideline:
Apps may not include buttons, external links, or other calls to action that direct customers to purchasing mechanisms other than IAP.
What's the scope of this requirement? Does it only apply to payment to
unlock features or functionality within your app or all payment behavior? Do online payment apps (Line Pay, Alipay) violate this guideline? After all, their sole purpose is to direct customers to their own payment mechanism.