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. :(
I also have an older Ipod touch, I was trying to find an audiobook player to use it to listen to books, I couldn't find any that was compatible.
You should be able to get pretty exact numbers for your app in AppStore Connect. The results might surprise you.
Typically we have way faster adoption in our apps than the trend. YMMV.