With the new version of start.me, we've decided to discontinue supporting the iPhone app. This way we hope to make your start.me experience more seamless with the website. Our bookmarklet offers essentially the same functionality.
For more information about the use of start.me on a mobile device, read this article https://blog.start.me/updates/new-mobile-experience/.
Even though we've stopped actively supporting the iPhone App, you can still download it in the App Store.