In Auto Renewable In App Purchase, how could the App's server be notified, if user toggle's off the Auto-Renew button in Settings?
Your App's server only receives info about the last payment from Apple's API.
If a user toggles Renew automatically to Off, it is not reflected in the API.
Hence, there is no means a developer can figure for sure when an auto-renewing subscription will end until it actually ends.
Also, there is no need for you to manually revoke the functionality before the period ends because the user has already paid for the subscription duration.
Only, case where you might need to revoke it manually would be when you need to sync the features across platforms (across web browser/mobile App/Desktop App with same account).
Cross-Platform Considerations
Product identifiers are associated with a single app. Apps that have both an iOS and OS X version have separate products with separate product identifiers on each platform. You could let users who have a subscription in an iOS app access the content from an OS X app (or vice versa), but implementing that functionality is your responsibility. You would need some system for identifying users and keeping track of what content they’ve subscribed to, similar to what you would implement for an app that uses non-renewable subscriptions.