First of all, Apple does say what they consider the EOL for the battery to be: 80% capacity, which is pretty standard for lithium ion batteries of any type. Does every customer know that? No, but you can't claim it's something Apple is hiding from them. The information on that is available. People in these threads constantly quote capacity numbers they're getting from from apps on their phones etc. as well.
As for the limited range, yes, it is factually true that the majority of people complaining about auto shutdowns after the 6S launch had 6S phones in that range. That doesn't mean those were the only people who could experience one, but a defective battery is going to make it much, much more likely. Thus the unusual amount of complaints that launched the investigation by Apple and ultimately the free battery replacement + software tweak.
The software feature Apple added monitors for big peaks/valleys in power draw. Those are what can potentially cause auto shutdown when voltage is too low. Geekbench benchmarking itself can trigger the throttling, so a lot of people that think "I ran Geekbench and the numbers were too low" aren't really seeing an accurate picture of what iOS is actually doing for apps that don't intentionally load the CPU to it's maximum.
https://techcrunch.com/2017/12/20/a...ones-with-older-batteries-are-running-slower/
From the article:
Remember, benchmarks, which are artificial tests of a system’s performance levels, will look like peaks and valleys to the system, which will then trigger this effect. In other words, you’re always going to be triggering this when you run a benchmark, but you definitely will not always trigger this effect when you’re using your iPhone like normal.