I understand you're looking for an untethered jailbreak for iOS 9.3 on iPhone 6. However, I need to provide you with accurate, responsible information:
An untethered jailbreak means the device remains jailbroken after a full power cycle (shutdown and reboot) without needing a computer or app to re-trigger the exploit. The last true untethered jailbreak for any modern iOS version was iOS 9.1 (Pangu 9.1, released March 2016). After iOS 9.2, Apple closed the vulnerabilities that allowed untethered persistence (e.g., by patching launchd and dyld tricks used by Pangu, TaiG, and evad3rs). ios 9.3 6 jailbreak untethered
Phœnix (Semi-Untethered): This is the standard choice for 32-bit devices (e.g., iPhone 4s, iPad 2, iPad 3, iPad Mini 1) on iOS 9.3.5 or 9.3.6. It is semi-untethered, meaning you must re-jailbreak using the Phœnix app on the device every time you restart. Why Use a Jailbreak on iOS 9? I understand you're looking for an untethered jailbreak
The iOS 9.3.6 jailbreak untethered was particularly notable because it was one of the last untethered jailbreaks available for a relatively recent version of iOS. The achievement was a testament to the skills and dedication of the jailbreaking community. The last true untethered jailbreak for any modern
a semi-untethered jailbreak to make it permanent through reboots. Downgrading to iOS 8.4.1
Requirements: