https://techcrunch.com/2018/10/05/most-ios-devices-now-run-ios-12-according-to-mixpanels-data/

Analytics company Mixpanel is currently tracking the install base of iOS 12. And the latest version of iOS is quite popular as it’s already installed on roughly 47.6 percent of all iOS devices. 45.6 percent of devices still run iOS 11, and 6.9 percent of iOS users run an older version.

Adoption rate is an important metric for app developers. With major iOS releases, Apple also releases new frameworks. But developers still need to support old versions of iOS for a little bit before moving entirely to newer frameworks and drop support for old iOS versions.