Valve pushed SteamOS 3.9.4 to the Beta and Preview channels late on October 21. The top fix is for the Steam Machine, where firmware updates would occasionally fail to apply.
The update also cleans up two regressions from earlier 3.9 builds. Some apps were running behind Steam's loading screen (Valve names A Plague Tale: Innocence), and screenshots were being saved at 720p even when the display and game were set to a higher resolution. Both are fixed. Valve also fixed a bug where changing the time zone could block switching to Desktop Mode until a reboot, cut down on audio clicks from the 3.5mm jack with sensitive earphones or speakers, and revoked certificates for certain certificate authorities with insecure private keys.
On the developer side, there's more work on hibernation, but Valve has temporarily disabled hibernation on Steam Deck because of some outstanding driver bugs. Hibernation is still a preliminary feature in 3.9 and isn't exposed in the Game Mode UI yet. The release also continues work on the experimental dirlock disk encryption support.
The notes cover every change since the current stable SteamOS 3.8, so the rest of the 3.9 beta is still here too, including Plasma 6.7.3 in Desktop Mode, a kernel rebased on Linux 7.2, wpa_supplicant as the default Wi-Fi backend on new installs, and preliminary Legion Go 2 brightness control. To try it, go to Settings > System > System Update Channel.