The most important part of Apple's informational letter was an offer towards the very end: Apple would reduce the price of an out-of-warranty iPhone battery replacement, from $79 to just $29, "for anyone with an iPhone 6 or later whose battery needs to be replaced, available worldwide through December 2018."
And so, if you own an iPhone 6, iPhone 6S, iPhone 7, or any other phones made after that, and are experiencing battery issues — maybe it's draining faster than it used to — head to an Apple store before December.
A small anecdote for you: My fiancée owns an iPhone 6S and had been experiencing battery issues for months. She'd constantly need to recharge her phone at work and at home, and felt like it wasn't always this bad. So a couple of months ago, we visited an Apple Store, where an employee measured her phone's battery life and found the degradation to be at about 83%. Apple says it will offer to replace batteries when battery degradation reaches 80%, but the employee gave my fiancée the option to replace it right then and there for $29. So we did that, and walked around the mall for a few hours while we waited. It was worth the wait: Since that visit, she's seen a noticeable improvement in her phone's battery life, and no longer needs to charge it throughout the day.
So if you're experiencing anything similar, go visit an Apple Store before December 31, and pay the $30 to get your battery replaced. You'll be paying more if you choose to wait.