Note: This will fail 80% of the time if the app uses APIs introduced after iOS 7. You will need a disassembler (Hopper or IDA) to fix API calls, which is an advanced reverse-engineering task.
Once you have your patched IPA, use one of these tools to sideload it: ipa files for ios 712 patched
As the years go by, the utility of patched IPAs shifts from productivity to preservation. The iPhone 4 running iOS 7.1.2 is now becoming a dedicated emulation machine. Patched IPAs for RetroArch, PPSSPP (PlayStation Portable emulator), and GBA4iOS are highly optimized for iOS 7, offering a gaming experience that rivals modern devices without the distraction of notifications. Note: This will fail 80% of the time
You might ask, “Why iOS 7.1.2?” This isn’t just a random number. iOS 7.1.2 was the final major update for the iPhone 4 and the last version before Apple introduced significant security changes in iOS 8 and 9. The iPhone 4 running iOS 7
Software history is dying. Millions of iOS apps from 2012–2014 have vanished from the App Store because developers didn't update them for 64-bit (iOS 11). For example, the original Flappy Bird installed via a patched IPA on an iPhone 4 running 7.1.2 is currently the only way to experience the game exactly as it was on release day. Museums and digital archivists argue that patching IPAs for legacy OS versions is a form of digital preservation, not commercial piracy.
To install patched IPAs on iOS 7.1.2, you generally need a way to bypass Apple's modern security restrictions.