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.
Do you legitimately think software you run does not know what hardware it’s running on?
If so, I can’t help you.
Local information for locally run software =/= phoned home to valve for whatever usage they want at all times for all purposes unless you have a source saying otherwise.
Do you have one?