I have been using a $100 temporary Android phone while waiting for my fancy phone to get replaced under warranty and the experience with the stock OS is just so so bad. Like barely usable most of the time. Even the GPS and wifi don't work well.
Most people worldwide use cheap Android phones on old versions of the OS and this was a good reminder that just because my mobile app/site works well on my fancy phone, it might work hardly at all for most people.
For starters here is an estimate of the most popular Android OS versions seen in the wild across the web:
https://gs.statcounter.com/os-version-market-share/android/mobile-tablet/worldwide
More than 50% of Android users are on Android 10 or below. Android 10 was released in 2019.