Was the Haptic engine included as part of the iPhone's Home Button ever made available for use by developers? When the iPhone 7 was released back in 2016, rather than the mechanical Home Button that had been included in previous generations, Apple replaced it with a Haptic version. At the time, mentions were made of using this new version of the Home Button to deliver notification alerts, as well as an API being introduced to allow developers to interact with the Haptic engine within the button (see https://www.theverge.com/circuitbreaker/2016/9/7/12828652/apple-iphone-7-home-button-removed-force-touch for more details). Just out of interest, was this API ever made available to developers - and if so, are there any examples of apps that made use of it?
Yes, Apple made that API available in iOS 10 (released in 2016). The main source of documentation is contained within the documentation for the UIFeedbackGenerator class:

https://developer.apple.com/documentation/uikit/uifeedbackgenerator

Many apps make use of it. For example the Halide app uses it to tell you, when you have the phone perfectly in "sync" with the grid. The Reddit app does it when you vote on a comment. The NBA 2k14 app uses it to indicate when the ball is touched. And many more apps/games exist that use haptic feedback.