This firmware was particularly notable in the modding community because it patched the WebKit exploits used by earlier jailbreak methods. If you're considering homebrew, 9.00 is generally not the version you want (versions 5.05-7.55 were preferred).
The update introduced several quality-of-life improvements aimed at bridging the gap between the PS4 and PS5 ecosystems: ps4 9.00 firmware
While the update was mandatory for online services at the time, some users reported critical system errors during its launch: How to Jailbreak Your PS4 on Firmware 9.00 or Lower! This firmware was particularly notable in the modding
: Running custom software, emulators, and media players not officially supported by Sony. ps4 9.00 firmware