Federico Viticci, writing for MacStories.net:

The f.lux team has built an iOS version of the app – unfortunately, they can’t release it publicly due to App Store restrictions. So, they’ve come up with a beta version that anyone can install with a free developer account and Xcode on the Mac.

This is interesting, but obviously presents certain dangers, since you’ll be running code that has not been vetted by Apple.

Imagine if, like on the Mac, Apple provided a framework to distribute and install iOS apps outside of the App Store with some security in place and a UI to manage sideloaded apps. Until a couple of years ago, it seemed obvious that it would eventually happen on iOS too.

Here, Federico is referring to Gatekeeper. From Apple’s official Gatekeeper page:

For apps that are downloaded from places other than the Mac App Store, developers can get a unique Developer ID from Apple and use it to digitally sign their apps. The Developer ID allows Gatekeeper to block apps created by malware developers and verify that apps haven’t been tampered with since they were signed. If an app was developed by an unknown developer—one with no Developer ID—or tampered with, Gatekeeper can block the app from being installed.

Not sure why this approach couldn’t work on iOS. Perhaps it’s a level of control that Apple is unwilling to relinquish.