According to Apple's App Store support page for developers, iOS 12, its latest mobile operating system now powers 78% of devices that introduced last four years, while 75% of all devices have updated to iOS 12, and 17% of devices continue running iOS 11 in both charts, the rest of it is earlier iOS versions.
The last time that Apple updated it is on December 4th of 2018, where iOS 12 has an installation rate of 70 percent on all devices and 72 percent on recent devices, this is almost identical to iOS 10's adoption rate, with the OS installed on 76 percent of active devices by January 5, 2017. Meanwhile, iOS 11 was suffered from much lower popularity, it was installed on just 65% of devices on January 25, 2018.
Most people are updating to iOS 12 is due to the fact that it brings huge speed improvements, especially on older devices like iPhone 5s or iPhone 6. Also, there are fewer bugs than iOS 11 once had, though it did have a couple of bugs, like poor cellular performance, other than that, no major issues whatsoever.
When it comes to Android distribution rate, the latest Android Pie was nowhere to be found, though it should be noted that the data hasn't been updated since October 26, 2018, most were running Marshmallow, Nougat, and Oreo by then.