The Vulkan 3D sparse image support for RADV was worked on for the benefit of the Zink OpenGL-on-Vulkan implementation. However, the 3D sparse image support also has use by VKD3D-Proton as the D3D12 over Vulkan implementation used by Steam Play.
The RADV 3D sparse image support was worked on by RADV co-creator Bas Nieuwenhuizen. The Vulkan sparseResidencyImage3D feature is enabling for Polaris GPUs and newer ( update: A patch landed a short time ago limiting it now to Vega and newer, due to broken Polaris behavior for the Vulkan CTS) while residencyStandard3DBlockShape is enabled for GFX9/Vega and newer with this RADV code for next quarter's Mesa 22.3 release.
This functionality is for 3D textures backed by sparse memory where only certain regions/portions are actually backed by video memory. This merge request that landed last night in mainline has all the details for those interested in RADV 3D sparse image support.