You have the cause and effect reversed. The speed of iPhone 6S dropped by 60%. That means A9 dropped to the speed of A6. iPhone 6 went from 1.4 GHz to 600 MHz. People definitely notice that in day to day use. People realized how slow their phones had become and ran Geekbench to confirm.
I don't think Apple slowed down old iPhones for the sake of slowing them down. But they were wrong and heavy handed. Apple "fixed" an unexpected shutdown issue by hiding the fact the battery needed replacement. Apple significantly reduced the performance because it only benefits Apple if the phone is slower.
If your car's engine unexpectedly shuts down on the highway and the manufacturer reduced power output by 60% as a fix, you'd probably want to know. Apple was completely silent on this issue until someone found out.
With Apple the “fix” is always in.
Was using an iPhone 16s + until the October 2023 release of iOS 15.8
The upgrade, which was as a security update, turned my perfectly functioning iPhone into eTrash. So much for the environment.
Areas where the iPhone functioned with only 1 or 2 bars now drops to NO SERVICE.
Apple’s and the carrier’s solution is to upgrade the phone and be locked into a 3 year contract.