Apple has updated its App Store Developers page to reflect that iOS 13 is now installed on 55% of devices introduced in the last year and 50% of all iPhones out there. As for iPadOS, it is running on 41% of iPads launched in the last four years, with overall 33% iPads out there running the OS.
Apple has also broken the user base of iOS 13 and iPadOS 13 into two, though this makes sense since they are now two completely different OSes and were launched at different times of the year. This also explains as to why the adoption rate of iPadOS 13 is lower than iOS 13.
The iOS 13 adoption rate puts it roughly on par with iOS 12 which was also running on around 55% of iOS devices around the same time last year. Apple released iOS 13 on September 19th and has since then rolled out multiple updates to fix a bunch of bugs and stability issues.
Apple has calculated the numbers by calculating the number of devices running iOS 13 or iPadOS that visited the App Store on October 15, 2019. The adoption rate ofiOS 13 and iPadOS 13 will likely jump even higher once Apple releases iOS 13.2 towards the end of this month. iOS 13 has been a relatively buggy release and Apple has been continuously rolling out updates to fix the issues. This might have deterred a lot of users from not upgrading to it.