"Just the OS updating" is not insignificant. Software developers, in general, are not known for making sure latest versions of their software run smoothly on older hardware.
Also, performance on iPhones is throttled when your battery is very old. There was a whole class-action lawsuit about it.
Conspicuously, Apple just so happened to pick the one that encouraged people to upgrade the entire phone. You know, an entire phone that is otherwise functional without arbitrary restrictions by the OEM.
https://support.apple.com/iphone/repair/battery-replacement
There is no “DRM” in their battery.
If you had the wherewithal, you could do it yourself
https://www.ifixit.com/Guide/iPhone+6s+Battery+Replacement/5...
The units weren’t “faulty” all batteries degrade over time.