So you sign up for... a free trial(?) in the app, then you get hit with a "upgrade your subscription" email? It seems like an awkward flow, but maybe it will be good enough for developers.
Want to cancel a subscription? Cool, just do it through iPhone’s Settings. No questions asked. There. Done. No “call our customer line/chat to cancel your subscription”. Want to renew it? Same place. Prices are clear and no “small print” non-sense.
Most anything else is user hostile. Why that might be, and why it’s popular to support doing those hostile things to users, makes you go ‘hmmm’.
You’re left with running after the users to close their iOS subscriptions if they cut ties with your service.