It would be nice to be able to install ChatSecure 4.3.9 on it, deployment target is set at iOS 9 for that release. I believe if you go to 'Pricing and Availability' section of appstoreconnect, at the bottom of the page there is Last-Compatible Version Settings section where 4.3.9 can be checked and then available for download on older devices.
They are all selected, all the way back to v1.0. You might need to make the purchase on a newer device and then go into your purchase history to download.
I have an old iPhone 4s with iOS 9.3.6.
It would be nice to be able to install ChatSecure 4.3.9 on it, deployment target is set at iOS 9 for that release. I believe if you go to 'Pricing and Availability' section of appstoreconnect, at the bottom of the page there is Last-Compatible Version Settings section where 4.3.9 can be checked and then available for download on older devices.