I've searched in the last few months and found apps that had screenshots that were clearly from iOS 4 or 5.
David Smith said he had been working on a piece before this was announced (1) and believed about 50% of apps never got updated.
Also the fixes to stop things from being named "Candy Soul Saga Crush Game Match Three Slots For The Win Pokemon Insurance Dating Fun Free Time" is a serious improvement too.
Random idea based on other stuff I saw app developers talking about on Twitter the other day: is this the first steps in going 64-bit only next year? If the apps haven't been updated they can be pulled before iOS 11 comes out and then OS update wouldn't get blamed.
1) https://twitter.com/_DavidSmith/status/771405838607089665