Juli Clover, MacRumors:

Apple in July 2021 began offering beta firmware for the AirPods Pro, with the software available for Apple Developer program members.

I don’t recall past AirPods firmware being distributed in this way.

‌AirPods Pro‌ firmware betas are limited to developers and are quite tricky to install, with an installation guide available below.

And:

Installing the firmware in an unauthorized way can put the ‌AirPods Pro‌ into an unusable state that necessitates an out-of-warranty repair, so non-developers should not attempt to install the software.

And, most importantly:

Apple says that once the ‌AirPods Pro‌ firmware has been installed, it’s important not to put un-updated earbuds into the charging case because they may be updated with the firmware.

This firmware features “FaceTime Spatial Audio and Ambient Noise Reduction”.