There is no possibility for iPhone 7 and iPhone 6S dropped at same time. You need to remember, the new iPod Touch which launched this year, shares identical hardware specification with iPhone 7. Also the iPad 6th generation also has similar specification as iPhone 7.
iPhone 5S is exception, because iOS 12 itself is exception. iPhone 6, launched with iOS 8, dropped with iOS 12, only gets 4 years updates. Which far less than your 6 years OS updates.
iPhone 6S only gets iOS 14, if iOS 14 happens to be one of iOS 12 types of release. Otherwise, it is in its last lag.