Testing iOS 9 app with TestFlight (Feb 2021)

I have a library that I've written that I want to support iOS 9+ so I've created a new application and submitted it for TestFlight. However, on the iOS 9 device that has been freshly installed with iOS 9.3.6 when I go to download Test Flight, it says

"Unable to Purchase"

"TestFlight" is not compatible with this iPhone.

Is there any way to get iOS 9 apps tested on a physical device in Release mode, either through TestFlight or any other means (other than jail-breaking)
Testing iOS 9 app with TestFlight (Feb 2021)
 
 
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