Most smartphone users have at least a few apps that they've installed, played around with for a day or so, and then entirely forgotten. These apps are using up precious storage and CPU cycles, so Google's been working on a way to get rid of them in a more subtle way. It's called app hibernation, and we've heard about it as an Android 12 feature for a while. Apparently it's live in the Android 12 beta released yesterday.
If an app is totally unused for an extended period of time, Android 12 will revoke any permissions it's been granted (as was already the case), then remove it from active memory and storage, and won't allow it to perform jobs in the background or receive push notifications. It should be a small but vital way for Android devices to keep running smoothly, even if the user is downloading more apps than they really need. The app will automatically come out of hibernation if the user manually launches it.
If for some reason you want to make sure that an app isn't flagged for hibernation (say, an important Discord channel that only updates once every few months), developers can direct users to the Apps section of Android's Settings menu to turn it off.