Just like almost every Android version, there’s some change to notifications, even if it’s just a minor improvement and not a full-fledged functionality. Which is the case with the latest Android 12 DP3 update.
The re-design of the notification cards isn’t big as it just relocates the notification count for stacked cards. For instance, in Android 11, if you have more than one notification from a certain app, a badge with the notification count would appear in the lower-right corner of the stacked cards. Whereas Android 12 moves the counter right next to the expand arrow in the upper-right corner. The badge is even highlighted in a corresponding to the app’s icon color.
This change not only improves visibility but also saves up some space for text preview on the card. But as it’s usually the case with developer previews, everything is subject to change, so the current design may not end up the same upon official release of Android 12 in a few months.