While the AirPods Pro is connected to the iPhone via Bluetooth, there are some surprising things that are still in use for the 10-year-old iPhone 3GS. Twitter user Zed, a former Apple Store genius, says the AirPods Pro can be perfectly paired and used with the iPhone 3GS, and noise reduction is fine. The noise reduction feature spedgin is related to the hardware of the headset and has nothing to do with iOS.
The iPhone 3GS is the third generation of the iPhone, which went on sale in 2009 and went out of production in 2012. The latest version of the iPhone 3GS is iOS 6.1.6, released february 21, 2014.