Valve released SteamOS 3.9.3 to the Beta and Preview channels on October 16. To opt in, go to Settings > System > System Update Channel.
The changes new to 3.9.3 are fairly small. Valve lists a fix for performance problems in STAR WARS: Galactic Racer after resuming from suspend on Steam Machine, a fix for an issue that could lead to GPU errors in The Crew Motorfest, a fix for audio problems when hotplugging certain monitors in a multi-monitor setup, and improved support for the Steam Frame wireless adapter.
The notes also cover everything in the 3.9 branch since the current stable 3.8 release, and that's where the bigger changes are. Desktop Mode moves from Plasma 6.4.3 to Plasma 6.7.3, and the kernel is rebased on Linux 7.2. For other handhelds, Valve fixed a system crash when launching Forza Horizon 6 on the Legion Go 2 and added preliminary Legion Go 2 brightness control. It also fixed certain games, including ELDEN RING and DARK SOULS III, failing to run on recent Intel Panther Lake CPUs, and worked around a firmware issue on some ASUS devices that caused excessive fan speed after waking from sleep. There's preliminary hibernation support too, though it isn't exposed in the Game Mode UI yet.
My take: the 3.9.3 fixes themselves are minor, but the 3.9 branch as a whole is a really good sign for anyone running SteamOS on something other than a Steam Deck. It is still a beta, so I'd only jump in if you're okay with the occasional bug. Are you on the beta channel, and how has 3.9 been on your device? Let me know on X or in the YouTube comments.