Apple has always been proud of the install penetration rate of the latest version of iOS, but the latest iOS 15 is inferior to the previous versions, and the penetration rate between devices within 4 years of launch has only been 72%.
Apple recently updated the install penetration rate of iOS 15 so that app developers can use it as a reference, and the data shows that 72% of devices within 4 years of launch have already installed iOS 15, while iOS 14 still has 26% of devices installed. The penetration rate of iPadOS 15 installed in devices launched in the past 4 years is only 57%.
While it's already pretty good compared to Android, previous generations of iOS have performed better, such as iOS 14, which had an installed penetration rate of 81% in December 2020, and iOS 13, which also had a 77% penetration rate in January 2020. Apple has seen this, previously changed the system update prompt display mode, no longer let users choose to stay in iOS 14 but more emphasis on updates, currently to get the latest security updates can only upgrade to iOS 15.
Source: MacRumors