Just an idea ... if you already checked the game files integrity, the problem might not only come from the game itself but maybe find its source in your documents/assetto corsa.
You might try saving this folder or renaming it documents/assetto corsa bak .... and launch the game so a new folder is created and check if this slider is back.
If it's not back, you'll know for sure the problem is not there and replace the new created folder by renaming the "bak" one after deleting the new created one....
If the slider is back in game .... you'll just have to copy your main important files back in the new documents/assetto corsa step by step with checking game after each step.... so you can find where is finally the problem cause.