iOS Subscription Offers can only be redeemed in sandbox after a subscription has expired. This is related to the fact that product changes don't work in sandbox. In production, an active or lapsed subscriber will be able to redeem an iOS Subscription Offer.
Apple: Implementing Subscription Offers in Your App
Apple: Setting Up Subscription Offers
WWDC Video: Subscription Offers Best Practices
RevenueCat: Signing iOS Subscription Offers with RevenueCat
If you see anything that needs to be fixed or have anything to add, please submit a pull request!