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.
I was referring more to just like Windows, Linux, MacOS icons. Not specific distros. As a Windows user for instance, Linux issues from any distro probably don’t matter to me.