I don't think there's anything in this injunction that stops Apple from building a mandatory subscription-cancelling API, which I wouldn't be surprised if they're actually already working on due to earlier rulings this month. In other words, you might be able to subscribe to FooCalendar using their Stripe API system, but FooCo has to make sure that your FooCalendar subscription shows up in your list of subscriptions, maybe under a "Non-Apple Subscriptions" list, that still lets you cancel with one tap -- or at least one tap to take you to that app's subscription management page.
(I'm sure there will be some people upset at that kind of interfering overreach, but it's the kind of interfering overreach most of us would actually like.)
Why would Apple make and provide something like that? They won't get anything for providing such an API to make the billing experience nicer for their competitors. It's directly in their interest to have as many scam billing providers as possible so they can point at them and tell us that this is what they protecting us from.
(I'm sure there will be some people upset at that kind of interfering overreach, but it's the kind of interfering overreach most of us would actually like.)