What design defect? Batteries deteriorate as they age and the found a way to extend the life of old devices by detecting and Preventing spikes caused by those old batteries that would end the device life altogether. You just have to love how the are forced to apologize for allowing customers to keep their devices longer. Instead of letting the die naturally. People are getting weird these days. It’s like the reason Mcdonalds doesn’t allow managers to feed the homeless with food they are throwing away at the end of the night. They would be legally responsible if the choked on a French Fry the gave them.
There was an official battery recall but apple said it was only a small amount of affected iPhone models at the time. The theory is there were way more affected batteries but apple instead put out the throttling feature to cover up the manufactured defects not under the recall.