At the moment, there is one available DirectX version for Classic, and it's DirectX 11. If anyone is ever confused as to what setting is and isn't available, you can just open up the in-game settings and look at the API drop-down to see what's listed there.
So, with only one available option to choose from, using a commandline parameter in an attempt to change the API is not going to do anything special, and the game is obviously going to ignore it.
EDIT: While the above holds true, in general, there's further clarification in my next reply.
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