iPhone Battery Draining Fast

Super question. Expect that page in the near future. Until then, some of the causes that overlap with the ones here. Most of the time that's related either to lint in your charge port, a crummy USB cable or power brick, or the charge port assembly itself is bad. You might be better off heading over to the Answers Forum in the meantime. That way you can crowdsource and give the community some more details.

Battery life unfortunately is a very complex thing. It is affected by so many factors it’s hard sometimes to say what’s causing poor battery life or to advise ways to improve it. It could be so many things.

Although it is true to some extent; newer operating systems will introduce more features that require more power. In my opinion this shouldn’t make a huge impact. In fact some iOS updates have specifically been designed to address poor battery life on older devices. Depending on what your battery health and how you use your phone they may be right. But I am still always skeptical if a device is still on its original battery and it’s been in use a few years.