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Saw this quote on their forums: https://forums.robertsspaceindustries.com/discussion/comment/7581676/#Comment_7581676
Years ago we stated our intention to support DX12, but since the introduction of Vulkan which has the same feature set and performance advantages this seemed a much more logical rendering API to use as it doesn't force our users to upgrade to Windows 10 and opens the door for a single graphics API that could be used on all Windows 7, 8, 10 & Linux. As a result our current intention is to only support Vulkan and eventually drop support for DX11 as this shouldn't effect any of our backers. DX12 would only be considered if we found it gave us a specific and substantial advantage over Vulkan. The API's really aren't that different though, 95% of the work for these APIs is to change the paradigm of the rendering pipeline, which is the same for both APIs.