As I couldn't find the nice page about the IAP policy that I read and found quite clear ~4 years ago (you should verify, don't trust people on the Internet): the old rules prior to the Epic and other court cases would say "Only for those users that set up payment while using the app." From what I gather, the current rules would be more lax, but the old can then serve as an upper bound.

The main rules were:

1) A user making a payment that has any effect on app functionality (this is very broad) from inside your app must go through Apple IAP (paying 30% commission). There were some exceptions, but mainly think buying physical goods - Apple isn't forcing the use of IAP for EBay or Amazon retail.

2) You may not link to, or even mention other payment methods.

Really it comes down to: if there is any difference in what the app will do for a user before and after they pay money then that money paying must go through Apple and then Apple will give it to you (minus 30%). So what a lot of apps did is just open to a login screen. No sign up button, no link to your homepage, heck, no links anywhere. The downside is that users have to know about your app from at least one more channel than searching on the App Store, but that's not a very high bar. Probably less common if you're B2B, but for B2C a lot of them would pay their subscription through Apple if possible, the experience is great: one central place in system settings showing all your subscriptions with easy cancellation buttons and enforced standard refund/proration semantics.

In my limited experience, payments for such services are handled outside the iOS app. Simultaneously you're prohibited from directing users to make a purchase on your website and it's up to you to fit this square peg in a round hole, usually through cryptic messages which vaguely remind your users that they have to buy the subscription elsewhere, wink wink.

If you allow them to subscribe in the iOS app, then yes. Just don’t offer that option.