It does say in iOS that your battery needs to be replaced [1]. Everyone keeps saying they never told anyone about this but they basically announced it to the press with a statement (as well as the notice in iOS) when iOS 10.2.1 was released [2].
[1] https://support.apple.com/en-us/HT207453
[2] https://techcrunch.com/2017/02/23/apple-says-ios-10-2-1-has-...