The iPhone 4 is the newest iPhone to not get iOS 8. That's going all the way back to a phone released in 2010, nearly 7 years ago. There are definitely a few out there but I'm not sure it'd be more than half running <= iOS 7, especially when you consider how strong Apple's update numbers are.
In fact, according to a marketing company, as of March last year, only 37% of globally active iPhones were even as old as a 5S [1]. Even the iPhone 4S, which did get iOS 8, was on just 5%.
Even if that data is skewed towards newer phones due to the type of sites they market on, it's unlikely that anywhere near half of active devices are running a software version over three years old.
Also, the App Store offers you the last compatible version of the app if your device isn't up to date, and it's possible that that version of Snapchat is still capable of showing ads, although I don't have one around to test it.
In fact, according to a marketing company, as of March last year, only 37% of globally active iPhones were even as old as a 5S [1]. Even the iPhone 4S, which did get iOS 8, was on just 5%.
Even if that data is skewed towards newer phones due to the type of sites they market on, it's unlikely that anywhere near half of active devices are running a software version over three years old.
Also, the App Store offers you the last compatible version of the app if your device isn't up to date, and it's possible that that version of Snapchat is still capable of showing ads, although I don't have one around to test it.
[1] http://www.adweek.com/digital/data-37-of-global-iphone-users...