The latest software versions for iPhones and iPads were announced about half a year ago now, and have been found to be installed on about 70% and 60% of their compatible devices released over the last 4 years respectively. However, in the case of iOS, this figure is beaten by that of the preceding 14 version from a year ago.
https://www.notebookcheck.net/iOS-15-is-beaten-by-its-predecessor-in-terms-of-adoption-to-date.593266.0.html