Google isn't only regularly releasing monthly system updates to Android phones, it's also pushing so-called Google Play system updates to devices. They're independent from full system releases and have been created as part of Project Mainline, an effort to speed up updates to certain parts of the Android system. As reported by XDA Developers, the process has received a small makeover as part of the latest update, with a new percentage bar now showing the installation progress on the boot screen on Pixel phones running the Android 12 Beta.
It looks like the new progress screen shows up for everyone installing the latest Play system update that just started rolling out to phones. Once it's downloaded on Android 12 and you hit the restart button, your Pixel phone will restart as normally, but then display the progress it's making in percentage. It seems like at least all currently supported Pixel phones are in for this treatment, as XDA confirms it's working on a Pixel 5 while we got it to show up on a Pixel 3, pictured below.
The underlying code for the percentage progress was introduced to the Android Open Source Project (AOSP) back in February, though Google wasn't committed to shipping it at the time. Given that all Android manufacturers rely on the AOSP build in one form or another, it's possible that the progress estimates will also make their way to more phones once Android 12 rolls out more widely.