... Technically you can still write OpenGLES and ship ANGLE with your game, or take a similar approach with WGPU (The rust library) instead of using DX12 or even DX11.
... And technically DX9 is still supported in Windows, right? It's just deprecated, and I don't think it supports full borderless fullscreen either for low latency swaps.
... And isn't there DX9on12? That is, a project that emulates DX9 through DX12? Or did that project get canned?