Update: The iPhone 6s and iPhone 6s Plus have shipped with "trackpad mode" enabled using 3D Touch. It seems like that was the plan all along. Oh, iOS 9 beta, you tease.
One of the new iOS 9 features Apple announced for the iPad was "trackpad mode". Place two fingers down on the QuickType keyboard and it transformed into a trackpad allowing for highly accurate cursor placement and better control for text editing. It would be terrific to have the same feature on the iPhone.
Apple waited until iOS 3 (iPhone OS 3) to ship text editing but when they did, they nailed it. They nailed it so hard competitors are even now adopting similar mechanics for upcoming releases. Trackpad mode, however, takes it to another level.
Although it was never announced for iPhone, and never appeared in the public beta for iOS 9, it's a poorly kept secret that Apple at least experimented with universal trackpad mode support in the early iOS 9 developer betas.
There might be good reasons why Apple isn't currently offering trackpad mode for iPhone, including usability. When four-finger app switching gestures were introduced, they were iPad only as well and never made it onto the iPhone. Larger screens are absolutely better suited to more advanced gestures.
Yet two fingers don't seem like too many, and big and bigger iPhones like the 6 and 6 Plus are blurring the lines on what can and should be considered large screens. Also, like with the four-finger app switching gestures, Settings could allow the feature to be toggled on only by those who want it and consistently find it of value.
Regardless, it's an incredible concept, and I hope it appears on the iPhone, in one form or another, either in the final release of iOS 9 or in a subsequent update.
We've filed this issue with Apple's bug reporter: rdar://21969935