The iPhone 6 was released on the 19 September 2014^[1], which marks the beginning of its update period. On the 19 September 2019, the iOS 13 was released, a next version of the operating system, which not made available for the iPhone 6, thus leaving iPhone 6 on the the previous version of the operating system, the iOS 12^[2]. This date marks the end of what we consider its update lifetime.

These two dates are 5 full years and 0 full months apart, which gives it A tier.