If I have a web app allowing people to register and pay a recurring fee. Can Apple require that people using the iOS version app pay the subscription through the web store and then pay the Apple tax ?
No, this has never been the case. For example, you can buy a subscription to the Downdog app through thir website where it's SIGNIFICANTLY cheaper than via their iOS app. After this, you can continue using their iOS app as a fully paid subscriber just fine.
To be honest, I have no clue. When they have physical control, they can block whatever they want. When you enter an agreement with them, they can legally force things.
What they can not do - however - is act like they are the ones making the law, and saying stuff like they "let" you and me.
They are not gods and their only prophet was a Cluster B disordered hippy who is now dead.