According to the System requirements for iCloud article, iOS 11 is the minimum supported version, which could also mean that attempting to log in to anything older than iOS 11 will no longer work.
Logging in to iCloud on an iPhone 4s running iOS 9.3.6 after performing Erase All Content and Settings no longer works while doing the same on an iPod touch 5 running iOS 9.3.5 still works. I also managed to log in to iCloud on an iPhone 5c running iOS 10.3.3 after performing Erase All Content and Settings.
Both the iPhone 4s and iPod touch 5 use the A5 SoC but only 1 is blocked from logging in to iCloud.
Logging in to iCloud on an iPhone 4s running iOS 9.3.6 after performing Erase All Content and Settings no longer works while doing the same on an iPod touch 5 running iOS 9.3.5 still works. I also managed to log in to iCloud on an iPhone 5c running iOS 10.3.3 after performing Erase All Content and Settings.
Both the iPhone 4s and iPod touch 5 use the A5 SoC but only 1 is blocked from logging in to iCloud.