Valve has released a major Steam Client Update for Windows. While the changelog is relatively long, the main change is a new Big Picture Mode. The ageing UI has finally made way for the Steam Deck UI, which contains several Windows-specific changes.https://www.notebookcheck.net/Steam-Deck-UI-now-available-on-Windows-without-workarounds-courtesy-of-latest-Steam-Client-Update.690183.0.html
The Valve change log doesn't actually mention "Windows". This is because the update is available for ALL platforms, including Linux and macOS. It's not just Windows.