Is it possible to play and use HEIF/HEVC files on iOS 9 and 10 using some kind of library, player or something? or is this something exclusively for iOS 11?
In June 2017, Apple announced support for HEIF in macOS High Sierra and iOS 11 which were released in September 2017. So, you can not use HEIF/HEVC in ios 9 and 10 or in macOS prior to Sierra!
Reference : Apple documentation and Wikipedia !