iOS 9.3 was released yesterday to the public and I have installed it on my iPhone 5c. What I'm wondering is, why is Night Shift not available for my phone? Is it the architecture of the processor, something to do with the screen, or perhaps something else?
From Apple site:
Night Shift is available on iPhone 5s or later, iPad Pro, iPad Air or later, iPad mini 2 or later, and iPod touch (6th generation).
So I think It's related to 64bit processor architecture.
You can install F.lux instead using xcode. It's not very complicated and it doesn't require you to jailbreak your phone at all and it's not a hack : https://justgetflux.com/sideload/ You'll just have to download the project file from another website as they've deleted it following apple's demand to take it down. It works nicely on my 5C.
I might guess, based on insight from being a software developer: The f.lux page says that it is no longer available because it doesn't use the documented API and Apple doesn't like them getting around it. The new Night Shift feature is part of a new iOS release, so it must use a newly added API feature. That feature may be added for current devices only and it would be different to implement on older devices, and they did not put that work into it because it's obsolecent.