https://phys.org/news/2018-05-apple-refunds-paid-iphone-battery.html

Then, on Dec. 28, Apple said it would reduce the price of replacement batteries for iPhone 6 models or later from $79 to $29. Due to consumer complaints, Apple sped up its replacement—originally for late January—and regardless of the capacity of the older iPhone battery.

So if you are a customer who paid full price before Dec. 29 to replace an out-of-warranty battery, you are eligible for a refund if your battery replacement was done at an Apple store, Apple Repair Center or an Apple Authorized Service Provider. This offer is good for anyone who bought an out-of-warranty battery replacement from Jan. 1 to Dec. 28.