The adoption of iOS 11 has probably slowed down a lot as the next version of iOS (12) is due to be released in days time. Apple's last update to iOS 11 was 11.4.1, released in July with bug fixes.
With iOS 11 on 85% per cent of devices, 10 per cent are continuing to use iOS 10, and five percent using iOS 8 or 9. iOS 11 adoption has been slower than iOS 10's because of how buggy the update was.
Apple decided to delay a redesign to focus on fixing up bugs and performance problems, and iOS 12 dramatically improves stability too. Along with that, iOS 12 brings Screen Time, FaceTime improvements and more. Apple may be launching iOS 12 to the public on September 19th, with a full rollout by the 20th.