I have a non-consumable in-app purchase that displays the price dynamically and localized from the StoreKit response.
Say it is normally $9.99 and I change it to 4.99 in App Store Connect. How could I promote it as discounted 50% off in the app without hardcoding anything?
I don't believe there is anything in the APIs to handle this.
The only way to accomplish this now would be to create a new non-consumable that is specifically for the temporary sale and client logic could determine the sale discount %/amount. Post-sale, you could remove the sale non-consumable from sale and your app could handle that dynamically, if the sale product price is not returned by StoreKit, your app doesn't display the discount copy/UI.
But please do file a feedback assistant ticket with your enhancement request to support full support for temporary pricing and provide API level data for discount merchandising. http://feedbackassistant.apple.com