While there aren't any major new features or content additions in this update, Bethesda has made some tweaks to the game's UI and gameplay mechanics. Specifically:

Update 1.6.640 is a necessary evil. It fixes no major long-standing bugs (the unofficial patch still does that) and introduces no new quests or features. However, it modernizes the game’s backend for Steam Deck and future Bethesda integration. For the average player, it’s a smooth transition. For hardcore modders, it was a weekend of updating DLLs.

If you meant the for 1.6.640, here's the short summary (Bethesda's official notes):

This paper examines Update 16640 for The Elder Scrolls V: Skyrim, summarizing the patch’s key changes, analyzing its technical and gameplay impact, and evaluating implications for the modding community. Using a combination of patch-diff analysis, player telemetry (where available), and community feedback, the study identifies bugfix patterns, balance adjustments, and compatibility risks introduced by the update. Recommendations for players, mod authors, and developers are provided.