After installing the new iOS version 11.3, my iPhone 6 Plus is working again. It’s like having the original back. Much faster by a factor of 3-4 times. That’s a huge WOW for me, but also a huge loss of trust in Apple. They allowed the older iPhones to become borderline unusable, and did nothing about it until a whistleblower wrote an article about it. Shame on them, and on Tim Cook, for allowing the iPhone to become so degraded over time.
They did it so your device wouldn’t shut down randomly. The only mistake was not being transparent about it. That’s okay though because now we all get to know more about our batteries than we ever did. Everybody wins.