Update June 24, 2020:
At WWDC 2020, a new notification was introduced that informs you of refunds: https://developer.apple.com/documentation/storekit/in-app_purchase/handling_refund_notifications
Original answer:
I received a response from Apple Developer Relations today (Dec 6, 2018):
Hello Johannes,
In response to your question, unfortunately, there is no supported means to detect that the user has contacted Apple Care and received a refund for the In-App Purchase of a consumable item. The only option which I can refer you to is to submit an API enhancement request for an API to be made available for an app to detect that a refund was provided to a user of an In-App Purchase. Currently, this support is realistically only available to apps which offer auto-renewable subscription In-App Purchase.
You can submit the enhancement request using the Apple Developer Bug Report web page – http//bugreport.apple.com
As this is an enhancement request type issue, I’m going to arrange for this incident to be unbilled from your account for use on a future issue.
So there we have it.