As you see in the article you linked, it was ruled unconstitutional only because it violated the 15th amendment. That amendment makes it illegal to deny the right to vote based on race, color, or previous condition of servitude. It is still legal to deny the right to vote if someone is deemed not smart enough.
The literacy tests in question were designed from the start to be ambiguous, so that the same answers could be marked wrong for a black person and correct for a white person. Frankly, there is nothing that could convince me that the Republicans wouldn't try to repeat the same thing if given the opportunity.
