I got my iPhone 6s life extended 2 more years with a non official battery. It degraded faster than the original one but also the OS updates probably caused more consumption after using the same model for 6 years
Apple fixed that glitch: https://www.ifixit.com/Answers/View/802054/Unable+to+verify+....
You are correct that it's still technically possible to replace a battery yourself, if you don't mind the Shareware-style nagging.