I just checked <https://mixpanel.com/trends/#report/ios_13> and it shows iOS at 85%.

About a month ago, I started releasing versions of our app on iOS 13 in TestFlight (I wanted to use the new SF Symbols) only to discover there are a lot of iPhone6 models out there. My first reaction, was "c'mon, that's a 6 year old iPhone, time to move on!" until I found that Apple kept selling the 6 all the way through 2017. A lot of "budget" buyers purchased iPhone 6's up through that period. So I've come to be skeptical about these claims of adoption rate. I subsequently reverted to iOS 12. :(