If you click on Linux only and expand the details, SteamOS accounts for 31%. "Other" is the next biggest at 23%, and Arch (which SteamOS is built on) at 10%. Someone probably has made a comprehensive historical spreadsheet but the internet archives save the page frequently if you want to see trends. Even historically though Mac has commonly been known for being around 2%, at times more, it's only recently (like in the last couple years) that it fell below 2% and Linux passed it. SteamDeck did have a big impact for sure, though. I've seen estimates for around 4 million sold so far after three years, that's not bad. Putting some extra effort into your game's release to get it "SteamDeck Certified" seems worth it to me just to get that little bit of extra attention from those customers.