iPhone 6 iOS 11 Public Beta: Camera format
Hardware encode for HEIF requires an A10 chip, and thus is only supported on iPhone 7 and newer. This is why you don’t see the option to save as HEIF on your iPhone 6:



Hardware decode support starts with iPhone 6s and iPhone SE:


  For iOS, we have hardware decode on the minimum config of the A9 chip.




Introducing HEIF and HEVC - WWDC 2017

However since all Apple devices will support at least software decode, you will be able to share HEIF images natively (by iMessage, or any other sharing service that negotiates decode capabilities) with all Apple devices.