That's a plus, but the developer should also be able to offer the option in-app, which they can't.
Tin foil hat: Apple will not allow that feature so that you will need an Apple device to unsubscribe.
You can do so from your iCloud account on the web.
Oh I'm sure. We wanted to be able to automatically cancel your Apple subscription if you upgrade on Android or Web, but we can't. We do this for Android where we can programmatically cancel.
Part of the reason is that the entire purpose of the app store is not to be forced to give your private information to every Tom, Dick and Harry private vendor.