It seems odd to me that at the current state of technology you would develop the Vulkan pipeline to even support non-hdr rendering (or at least at any stage other than the final framebuffer). I am thus surprised that SDR is the default (and currently only) mode for HDR. IS there a technical reason (memory consumption from floating point render buffers, performance, …) why Vulkan is not HDR first?

While I don’t have the knowledge to accurately explain the current limitations on HDR output, I can confidently say that the pipeline is internally HDR up until the end, and has been (I think) since the project started.