The changes listed for this SteamOS 2.63 update are mostly dominated by package updates to fix the recent glibc security issue that's been making the rounds. However, aside from that, it also has a NVIDIA proprietary driver update.
This release switches to this week's NVIDIA 355 beta for those using supported NVIDIA GPUs, which means two days after the specification release, SteamOS now supports the exciting Vulkan 1.0, if you are a NVIDIA Linux gamer on Kepler or Maxwell.
Once the code has been mainlined in the weeks ahead, hopefully they'll pull the Intel Mesa Vulkan code going into Mesa 11.3-devel (not the upcoming Mesa 11.2). Meanwhile, we're still waiting on AMD's AMDGPU-based Vulkan Linux driver to come at some point in the not too distant future, but it still may end up being a number of weeks.
More details on this afternoon's SteamOS 2.0 beta update via this SteamCommunity.com thread.