True to one extent , A mod built/updated with DX11 will work fine in DX9 however a Mod built for DX9 will have some visual oddities from the old shaders when ran on a DX11 install.
In some cases the car appears as a blob of light when played in DX11.
This is what the APEX GT3 cars looked like on DX11 before they updated it ...
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& after the DX11 update with new shaders (and works perfectly in DX9)
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So DX11 is "reverse compatable" while older DX9 content wont look so good in DX11.
Some mods are not as bad but you will still see the glowing ,same goes for tracks ..