Valve landed a patch in Mesa 27.0 and to be back-ported to current Mesa stable series to satisfy more games running under Steam Play with VKD3D-Proton for working on AMD GFX8 GPUs like Polaris.
With the one-line patch merged today to Mesa Git, more newer games should run on AMD GFX8 GPUs: GCN 3rd and 4th Gen hardware from Tonga to the Radeon RX 400/500 Polaris graphics cards. The change is enabling Vulkan FP16 support by default for GFX8 hardware when VKD3D-Proton is running.
The Mesa RADV driver has offered the radv_enable_float16_gfx8 option for enabling FP16 format support on GFX8 hardware but hasn't enabled it by default since typically there aren't any performance gains and at times worse than using FP32 on these older Radeon GPUs.
Georg Lehmann explained though that newer Windows games are beginning to require FP16 support and thus enabling FP16 usage by default for VKD3D-Proton with these old graphics cards:
"Games are starting to require fp16, so expose it by default for D3D12. This fixes SSAO in Witcher 3 with the recent game update.We don't want to expose fp16 for all applications, because it's usually not faster and sometimes slower than using fp32."
The change is merged to Mesa for those still gaming on AMD GFX8 era hardware.