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.
Just have another option for multiple devices or types used beyond a certain threshold. Just like here they’re using a majority of time spent playing on a Deck to show that icon.