I'm guessing they'll have some criteria on that
When you take your phone in, they'll do a test on the battery's current capacity and status, using a custom app. If the battery has reported errors, or if the reported capacity is low enough (below 80% capacity remaining, as I recall), then they'll agree to replace the battery. Takes a couple minutes to run the test, and then about an hour to swap the battery.
(Caveat: I was last through this process on my old iPhone 5, about six months ago. I assume that the procedure and policies are basically the same for newer iPhones, and haven't changed substantively since then. My battery's remaining capacity was below 40%, and was reporting errors, when I had it replaced.)
As reported by so many others, this happened immediately after the iOS 11 update. And by immediately I mean within seconds of booting right after installing the update. I stupidly waited for 11.1 hoping it would address the issue rather than revert while it was still possible.
Apple will not replace my battery even under this new program, as it’s still considered healthy by their standard. However an iPhone 7 with 87% battery health is unable to run iOS 11 without frequent UI freezes or sudden power loss, as I’ve found out, due most likely to the fact that I’m running iOS 11 on hardware designed for iOS 10. I’d be okay with that if there was some way to revert, but there is not.