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.
Apple knew the batteries they had sourced were barely capable of supplying adequate power to the phone under load. As the batteries aged they rapidly got to the point the phones would just shut down unexpectedly even when the battery was showing 50% available charge. This was occurring during a period when the phones were covered by warranty or AppleCare. Rather than replacing the batteries under warranty/contract Apple took the step of throttling performance without giving customers adequate notification. The release notes only had a vague statement to the effect performance management was improved under certain circumstances.
Most customers realize batteries are going to lose capacity as they age, but they expect reduced runtime, not random shutdowns. What Apple did was find a way to avoid having to pay to fix the problem caused by their inadequate battery specifications.