This is such a silly metric. If you do it by hardware manufacturer instead of Android as a whole (which would make far more sense given Android's open source nature), you could instead say that "Previous Oreo Release is Installed on 90something% devices" as, unless a user is intentionally not installing updates, then every Nexus and Pixel phone from 2015 onwards would have Oreo installed. Similarly, every Pixel phone (as the cutoff is now between the original Pixel/Pixel XL and the last Nexus phones) would be in line to get the Android Pie update.
If you're looking at it, instead, from a development standpoint, that's something different. That's what you get for open source software that is at the mercy of hardware manufacturers. If the whole point of this object lesson is to point out how slow some otherwise compatible devices are at receiving a major Android upgrade release, then really, this should be a Samsung shaming party as Samsung is, by far, the slowest Android device manufacturer to release upgrades to their TouchWiz Android OS variant for Galaxy phones as well as security updates to the Android platform itself (which, nowadays, ARE separate from actual OS updates).
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Sadder is that Android 2.3 Gingerbread is still in the chart.
There's actually a decent reason for this. Android is installed on MANY devices, not just consumer smartphones and tablets (if any). There are a plethora of specialty devices (think barcode scanners that integrate directly with business management software or hubs for fancy conference room video/phone systems) that run a pretty much un-upgradable version of Android. I have come across two such systems in my IT travels. One was capped at Android 2.3 Gingerbread and the other was capped at Android 4.1 Jellybean.
In both cases, there was no Google play store or functioning dialer or messaging apps. Even e-mail and the gmail app were missing. But that's because Android wasn't there to be used as a consumer device OS wherein you install a bunch of apps and personalize your device. Instead they were solely being used as single purpose devices. And so long as those versions of Android were locked down and you were unable to venture out into the Internets or Google Play to install something nefarious, the fact that you were on an old version of Android did not matter one bit.
So yeah, plenty of devices out there probably still running Android 2.3 or 4.x. I guarantee you most of them aren't actual phones or tablets and/or have a reason why them being at that version and not newer actually matters.
Then again, another way to look at this is that Android 2.3 debuted in December 2010. The contemporary version of iOS at the time was iOS 4. Nothing capable of running iOS 4 is able to run any version past iOS 9 today. In fact, of the devices that could've ran iOS 4, only the Fourth Generation iPod touch, iPhone 4 and 3GS, and iPad 2 even made it past iOS 5, and only the latter-most of those devices even made it past iOS 7. At least Android 2.3 can be still made to do something actually practical in 2018.