Quick answer: Valve's latest SteamOS beta fixes a few game issues, and the wider 3.9 branch brings Plasma 6.7.3, Linux 7.2 and fixes for Legion Go 2 and Panther Lake.
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.