Recently we learned that Google's taking a familiar direction in Android 12: making the power button do something besides powering off your phone. By default pressing the power button will open Google Assistant, mirroring similar functions from the iPhone and Samsung phones. It looks like Google recognizes that this is a less-than-intuitive way to go about things, because it's building in a power down command to Assistant.
9to5Google spotted screens indicating that this option will be coming by examining the assets of an update to the Search app. Presumably it'll work on whatever version of Android you're using (so long as it can run the latest version of Assistant). The functionality isn't working at the moment, like a lot of new things introduced in the Android 12 beta.
Possibly more interesting: the screen indicates that if you leave the power button-to-Assistant functionality enabled, you can use power + volume up as an alternate power button, activating the familiar power menu with the option to turn the phone off or restart it. A bit of quick testing shows that this is also not enabled in Android 12 Beta 1, but it looks like it will show up later.