This issue is occurring on iPhone 5, 6, 6s. There are thousands and thousands of posts on it. Many of the issues are fixed by doing a factory Reset, or even a battery calibration, almost all fixes are “software or bits” related. In some cases ya, the battery is bad, but not even close to the majority. I did my homework on this issue, Apple has a serious issue they are unable to track down. Just do a search on “iPhone 6 shutting down randomly” in google, read the official blogs on the steps necessary to “fix” it and the thousands of replies from bewildered Apple fans. Sorry Apple Fan boy, I do know what I am talking about.
And there are approximately 800 million iPhones of those models in the wild, which means that after you discount for the ones that merely require a hard reset, which is always recommended after a major software upgrade to clear out mis-behaving apps, leaves at most a few hundred to a few thousand iPhones out of those 800 million or so, leaving a minuscule fraction of the entire universe of iPhones in the wild that may have a problem. That is far fewer than 0.1% of the iPhones in the wild that have such a battery problem. Ergo, it is NOT an iOS or universal app problem which could be easily identified if what you claim were true. As an engineer, you should be able to do this math easily.
"Thousands of complaints," especially when most of them are mere comments in reference to an original complainant, in a universe of over 1.1 billion devices does not translate to huge problems as you seem to be conflating this issue into. The statistics of these commentary forums have consistently been shown to have one complaint to approximately 80 comments from non-complainers, which should give you an idea of how many users are actually experiencing the issue enough to make an online complaint.