VanCoyld It's on the to-do list.
The thing about this filter is that it works well for Games, but not for Deals. On the Deals page, we can't just point our which games are Steam Deck Verified / Geforce Now compatible. We need to also look at the DRM. For example, RDR2 is Steam Deck Playable if you buy if from Steam, but the cheapest offer is usually for a Rockstar Launcher key, which isn't so easy to run on the Deck.
We will add this filter in the future, but we need some time to combine the Platform filter with the DRM filter.