Usually I'm against these frivolous class actions, but SOMETHING is going on with older phones. I know my experience is anecdotal, but my 6s was recently running fine until iOS 12.31 (now on 12.4) and all of a sudden my battery is draining super fast. Battery was replaced by Apple less than two years ago, and battery health reports at 88%. This is without any change in my app/usage habits.
They keep doing something that negatively effects the battery performance of my older phone, and it's really pissing me off.
Do you run 3rd-party apps? Do those apps receive regular updates that are installed automatically, or manually by yourself? You've already acknowledged that you've installed the latest iOS update, so clearly you're installing new software onto your phone willingly. So you can't claim that "nothing has changed". Software can have bugs that didn't exist in past versions. Age and environmental factors (eg. temperature) also affect our iPhones' battery capacity.
It may be easy to dismiss all of that and blame Apple for "doing something" to your device. We all know better.