Apple released its latest iOS usage report today, and unsurprisingly, iOS 11 is installed on 85% of all iOS devices. iOS 10 accounts for 10%, while 5% are on an earlier version. The report hasn't been updated since May, when 81% of iOS devices were running the latest version.
When iOS 11 launched, it dropped support for some older devices, such as the iPhone 5, iPhone 5c, and fourth-generation iPad. It seems likely that a sizable chunk of the devices that are still on iOS 10 are simply ineligible for an upgrade.
We're just weeks away from iOS 12 now though. Apple is holding its annual iPhone launch event on September 12, and beta testers will see GM seeds following the event. General availability will be a couple of weeks after. What's different this year is that iOS 12 doesn't drop support for any devices at all, so we may see even quicker adoption, as no devices drop from support and people with older devices upgrade to newer hardware.
In the meantime though, iOS 11 adoption has slowed to a crawl. It's not a surprise, as all iOS devices are eligible for updates on day one, as opposed to other platforms that use staged rollouts.