As of Wednesday Apple is no longer signing iOS 9.3.5, effectively preventing anyone already on iOS 10 from downgrading to iOS 9.
The company has also stopped signing iOS 10.0.1, making the latest versions of iOS — iOS 10.0.2, and 10.0.3 on the iPhone 7 and 7 Plus — the only releases anyone can install without unsupported workarounds.
Apple regularly stops signing updates as a security measure, but today's step could theoretically cause problems for people with older devices who are experiencing performance or compatibility problems with iOS 10. New versions of iOS typically don't perform as well on devices that are several years old — some tests have suggested though that iOS 10 is friendlier in this regard than past updates.
Separately, Apple has stopped signing tvOS 9.2.2 for the fourth-generation Apple TV. tvOS 10 introduced features like a dark mode, and upgraded Siri and HomeKit functions.