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.
It should also show icons for the OS as well. Since games can have dramatically different issues and performance depending on the OS sometimes. Having an icon showing a review is from a Linux machine compared to Windows for instance can help figure out if your system might be affected by reviews mentioning something that all seems to be from one type of OS.
Heck, I’d even take it a step further and have it include basic system specs for each review since Steam already gets that info.
I think OS is fine.
Making specs public is a privacy issue.