Quick answer: Valve's latest Steam client beta groups achievements into sets, makes the Linux client 64-bit and adds a Remote Play display option.
Valve has a new Steam client beta out, dated October 15, and it has a couple of things Steam Deck owners will care about.
The headline feature is Achievement Sets. Developers can now group a game's achievements into collections, like single-player and multiplayer, and DLC achievements can be split out from the main game too. Each set gets its own progress bar, and when you finish one, Steam adds a "set complete" trophy to your profile. According to Valve's notes, quoted by GamingOnLinux, if new achievements get added to a set later, your completion is preserved. GamingOnLinux points to Portal 2 as an example, split into single-player and co-op.
On the Linux side, the Steam client is now a 64-bit application and runs inside a Steam Runtime container, which Valve says should help it behave more consistently across distributions. Remote Play also gets a new "Display Style" setting so you can stretch or zoom the picture when the stream's aspect ratio doesn't match your screen. There are general stability fixes as well, and GamingOnLinux says Steam Deck and Steam Machine get the same fixes and improvements.