If the majority of a users playtime at the time of review was from playing on Deck, the review will have a Deck icon next to it.
The game in the screenshot is SAND LAND. You can view the reviews here to easily see some examples of this new icon.
Unfortunately there doesn’t seem to be a way to filter reviews to show Deck reviews only.
On the other hand, you have to keep your system specs up to date manually on protondb. I don’t know how many people keep their kernel and video driver versions up to date. Steam could pull the latest system configuration when I submit a review.