[Tutorial] "This App Is Incompatible With This iPhone" on iOS 6 and below: The problem + workaround

Tutorial

iOS 6 and below are affected by a problem where if the latest version of an app requires a hardware component that your device doesn't have, they will not serve up the last compatible version of that app as they normally would. In the majority of cases this is when the app you're trying to download now requires iOS 11.0+ (Netflix, YouTube, Twitter etc) and therefore demands a 64 bit CPU (arm64), a hardware component that no iOS 4/5/6 device has. As a result, iOS 6 and below won't offer the last compatible version of these apps and they can't be downloaded without carrying out additional steps.

The good news is this problem does not affect iOS 7 & above, which will offer an older version to download even if the latest version requires missing hardware, and will display the message "This version of [insert app name] is not compatible with this device. Would you like to download the last compatible version?"

***WORKAROUND***

Add arm64 as a hardware capability of your device so that it will once again serve up the last compatible version of iOS 11.0+ apps. Carry out the following steps:

1. Open iFile on your iOS 4/5/6 device and navigate to /System/Library/CoreServices/Springboard.app/
2. Locate the file [xxx]AP.plist (filename varies depending on your device, see list below)
3. Open the file with Property List Viewer and go to capabilities. Click the + icon in the bottom right corner, name it arm64 and set type to Boolean. Then click create, and toggle it on in the list.
4. Respring or reboot your device and you should now be able to download the affected apps as normal from your purchased history.

Thanks to u/iPodee for this information and easier solution.

Filenames by device:
iPhone 3G - N82AP
iPhone 3GS - N88AP
iPhone 4 - N90AP, N90BAP, N92AP
iPhone 4s - N94AP
iPhone 5 - N41AP, N42AP
iPod Touch (2nd Gen) - N72AP
iPod Touch (3rd Gen) - N18AP
iPod Touch (4th Gen) - N81AP
iPad (1st Gen) - K48AP
iPad (2nd Gen) - K93AP, K94AP, K95AP, K93AAP
iPad (3rd Gen) - J1AP, J2AP, J2AAP
iPad (4th Gen) - P101AP, P102AP, P103AP
iPad Mini (1st Gen) - P105AP, P106AP, P107AP

*Please note this tutorial is specifically intended to fix "This App Is Incompatible With This iPhone/iPad/iPod". If you are getting a "This app requires iOS x.x" message, it's because;
a) The app is not in your purchased history (download on a newer device or with iTunes 12.6.5.3 first)
b) The app vendor has removed older versions of the app from the App Store (e.g eBay, PayPal) - download and backup all the apps you need while you still can!
c) The app is fairly new and was never compatible with your firmware.

*Also note that these steps manually replicate most of what the "Checkmate, Store!" tweak does, which was released after I wrote this tutorial. However, the tweak has additional usage on iOS 3 as it enables the "download older version" functionality that exists in iOS 4 onwards.