Today, Apple quietly closed iOS 9.3.5 for iPhone 4s which means there is no iOS version available for iPhone 4s at all. Some people think that Apple is forcing users to upgrade device, while others surmised that Apple would release updates for devices that were not being signed earlier to include meltdown and Spectre.

Apple is letting users downgrade to iOS 10 that’s probably a bug. There is no info on why Apple is letting this happen, and thusly, it might just be a bug and Apple has quickly corrected a mistake.

Update 1: Apple reopens iOS 9.3.5 for iPhone 4s. You can install it on 3uTools now. 

However, after a while, it is said Apple reopens some older iOS versions for A5-A7 devices. But according to some users, they can not activate on iPhone 5s and iPhone 5c, then they have to update to iOS 11. Thus, please take risks downgrading to older iOS versions. Based on the current information, we don't know about the aim of Apple to reopen these iOS versions. At present, 3uTools does not suppport to restore these older versions, please wait for our updates. 

Update 2: iOS 10.2 , iOS 10.2.1 , iOS 10.3 are available on iPhone 6/6 Plus, iPhone 7/7 Plus! Not sure the activation problem. As shown in ipsw.me, there are more devices available to downgrade to iOS 10. Now you can downgrade to iOS 10 on 3uTools. 

Update 3: Apple closed those iOS versions, iOS downgrading party is over.