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.
Could have yeah, but I already find looking at distros and specs to be super helpful on ProtonDB. Of course it isn’t going to be 100%, but if every person who has a problem with a game is using a certain GPU or distro it’s usually safe to assume that something about that is causing the issue