Tools like Phoenix are the most common. They require an app to be "kickstarted" after each restart to reactivate Cydia and your tweaks.
For a or jailbreak museum piece ? This is legendary. An untethered iPhone 6 on iOS 9.3 is a time capsule – the last great untethered experience on a classic 4.7-inch display.
: Another semi-untethered option for 32-bit devices that offers high exploit reliability.
Once jailbroken (semi-untethered), install these to modernize the slow hardware:
Here is the brutal truth:
Security Vulnerabilities, Exploitation Methods, and Deployment Mechanisms for Legacy 32-Bit Devices Target Firmware: iOS 9.3.6 Architecture: ARMv7 (32-bit)