Quick answer: Valve released SteamOS 3.9.5 to all users on October 23, bringing the 3.9 update to the stable channel with Plasma 6.7.3, a Linux 7.2 kernel, better Steam Deck idle power and fixes for Legion Go 2, Steam Machine and ASUS devices.
After several weeks in beta, SteamOS 3.9 is out of testing. Valve released SteamOS 3.9.5 to all users on October 23, and it rolls up every change since the last stable version, SteamOS 3.8.
Desktop Mode jumps from Plasma 6.4.3 to Plasma 6.7.3, the Arch Linux base is updated, and the kernel is now rebased on Linux 7.2. Wi-Fi now uses wpa_supplicant by default for better compatibility with different networks, and Valve added fast Wi-Fi reconnect after sleep to that backend, which used to be an iwd-only feature.
There are plenty of handheld fixes. Legion Go 2 gets preliminary brightness control and a fix for a system crash when launching Forza Horizon 6. Valve also worked around a firmware issue on some ASUS devices that made the fans spin too fast after waking from sleep. The "Disable Frame Limiter" toggle now works with VRR turned on, frame limits no longer drop FPS below the target on certain external displays, and Game Mode no longer loses input if a launcher window gets dragged around.
Steam Deck owners get better idle power use when no audio is playing, which is the one new change in 3.9.5 over the 3.9.4 beta. Hibernation support is in the release but isn't exposed in Game Mode yet, and it still isn't enabled on Steam Deck because of an outstanding driver bug.
Steam Machine picks up fixes for firmware updates that sometimes failed to apply and for Remote Play not working when it boots without a display connected. Valve also fixed ELDEN RING and DARK SOULS III failing to run on recent Panther Lake Intel CPUs.