SteamOS 3.3 adds an "Achievement" and "Guides" pages to the Steam overlay, a scheduled night mode to automatically turn on the night mode at a set time, restoring the adaptive brightness toggle, expanded keyboard language support, initial IBus IME support on the desktop for various languages, a software update channel selector for choosing between stable / beta / preview channels, various performance and stability improvements, the Firefox web browser is now using the Flatpak version, improved virtual keyboard support in the desktop mode, various audio and Bluetooth fixes, and updated open-source Radeon graphics drivers for improved performance and compatibility. There also is a wide variety of bug fixes and other minor enhancements to SteamOS 3.3.
More details on all of the SteamOS 3.3 stable changes can be found via SteamCommunity.com .