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.
Steam already indicates which games will run on Linux, without any distro or configuration caveats. There’s literally an entire section of the store for “SteamOS & Linux” games, and games that support multiple platforms have the system requirements listed for each of those supported platforms.
We’re just talking about putting the icon for the platform they used on the review, to help you filter reviews that may be more or less relevant. Linux users complaining about Linux related issues aren’t relevant to Windows users for instance, and vice versa. Same goes for the 15 or so MacOS gamers I guess too.
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In those cases, they generally have the Ubuntu version that’s supported in the specs section