NV_dedicated_allocation is one of the extensions exposed by Valve's RADV Vulkan driver build but up until tonight wasn't found in mainline Mesa. The RADV support of NV_dedicated_allocation was crafted for Valve by David Airlie.
Seeing this support materialize in mainline Mesa for RADV isn't actually much of a surprise since NV_dedicated_allocation since this extension has been public since last summer. The other extensions used by SteamVR that aren't part of mainline Mesa/RADV include: VK_KHX_external_memory_capabilities, VK_KHX_external_memory, and VK_KHX_external_memory_fd. We won't see support for those extensions land until Khronos publishes an updated Vulkan specification that make them public. Both Red Hat and Valve are Khronos members and thus access to the working group data for unratified extensions and why for now Valve's RADV SteamVR driver build is binary-only.
VK_NV_dedicated_allocation is important for performance reasons and explained by the public documentation as, "This extension allows device memory to be allocated for a particular buffer or image resource, which on some devices can significantly improve the performance of that resource. Normal device memory allocations must support memory aliasing and sparse binding, which could interfere with optimizations like framebuffer compression or efficient page table usage. This is important for render targets and very large resources, but need not (and probably should not) be used for smaller resources that can benefit from suballocation."
So as of tonight in Git is VK_NV_dedicated_allocation support but on its own won't mean much of a difference unless there are any Vulkan games/apps explicitly using it but is more of an important stepping stone towards mainline RADV SteamVR support. Will we see the other Vulkan extensions published this week at GDC? Only time will tell.