There has been a lot of media coverage recently about a change that Apple made with a recent version of iOS (11.2) and how it affects some older devices. If your iPhone is more than a couple years old, you could be affected, so I wanted to take a moment to explain what's going on and why.
The iPhone uses a lithium ion battery. These batteries are a huge improvement over the rechargeable batteries of old, but like all rechargeable batteries they gradually lose capacity over time. When your iPhone was new, a fully charged battery might have been capable of providing 8 hours of use, but over time that could drop down so that even a fully charged phone might only work for 4 hours.
The battery doesn't just lose capacity, but the actual voltage being produced drops as well. Apple's recent processors require a lot of power (voltage) to be able to perform the heavy calculations needed to do their job. If they don't get the power they need they can stop functioning, which can cause the phone to crash, reboot, freeze, or otherwise misbehave.
People with older iPhones were experiencing issues where the phone might report 40% battery remaining and then suddenly power off and say it needed to be charged. They were also having problems with the phones rebooting or freezing. Apple was seeing a huge increase in the number of complaints of iPhones misbehaving in this fashion, so they tried to figure out a way to resolve it. Their solution was to make a change in iOS 11.2 that checked the capacity of the battery and, if it was lower than a certain point, to slow down the processor so it wouldn't require as much power. This prevented the phone from suddenly turning off, but it slowed the phone down instead—and if the battery was really crapped out, it slowed it down dramatically. People who felt like their phone was working fine (even though the battery was crap) suddenly found their phones became unusably slow with what they thought was a minor update of the operating system. Coincidentally, this change occurred right around the time that Apple introduced a new phone. Thus began the accusations that Apple was intentionally crippling devices to force people to upgrade. In fact, all that people needed to do was replace the battery on their old phone (which is $79 if you don't have AppleCare, and free if you do) and it would be back to running at full speed. Here's where Apple actually screwed up: they didn't communicate any of this to anyone, including the people working in their stores.
Anyone who has followed Apple over the years knows that customer communication is probably their biggest failing. Apple usually won't comment on a problem until it's getting worldwide media coverage, and they deny problems exist until this happens. It's not uncommon to find posts on the Apple Discussion Forums where hundreds of users complain about a problem, but Apple denies any knowledge of it (there were actually times when Apple would delete these discussions when they appeared, but thankfully that appears to have stopped).
If Apple has any plans on improving their communication with their customers, they certainly haven't communicated it.