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.
You can write the reviews from your PC still, and it will tag it as a steam deck review if most of your playtime is on deck.
As for typing on the deck, the OLED has a much more responsive touchscreen and typing is better on it. On the LCD deck I would stick to track pad typing though.