> new apps you might want to try on an older device
In that case, you borrow a newer device, sign in with your Apple ID, and download the app onto that device. Now that app will be in your purchase history and you can download the last compatible version for your older device.
All you have to do is get the app into your user account's purchase history. You can do that on a borrowed device and immediately hand that device back to the owner.
Apple keeps the OS current on it's devices for twice as many years as most Android device makers. Are we pretending that the sort of person who keeps the same device for up around ten years is the sort of person who constantly loads new apps?
In that case, you borrow a newer device, sign in with your Apple ID, and download the app onto that device. Now that app will be in your purchase history and you can download the last compatible version for your older device.