Firmware 9.00 is notable because it has a (using a USB exFAT exploit). Firmware 10.50 does not. Users on 10.50 sometimes want to go back to 9.00 to jailbreak.

file to a USB and installing it via Safe Mode is likely a scam or contains malware. Alternatives for Jailbreaking

Use software to "patch" the Syscon to tell the console to boot from the previous slot. Solder the chips back and trigger a system initialization. Why Version 9.00 is the Target

Reverting a PlayStation 4 from firmware to 9.00 is not possible through standard software menus or USB updates alone. However, it can be achieved through a highly technical hardware process known as firmware reverting . The Core Concept: Slot Switching