but it’s a single example
No, it isn't. In fact, the South wasn't even the first in the US to do this for the express purpose of disenfranchising a minority – more evidence that you aren't paying sufficient attention to the policy you want implemented. Some decades earlier, Connecticut implemented literacy tests to specifically (and successfully) disenfranchise Irish immigrants.
Even if you ignore that:
- Jim Crow-era literacy tests happened over a period of 75 years. That's not "one example"; that's a bunch of states over the course of nearly a century disenfranchising millions.
- It's so blatantly obvious once you examine that "one example"(TM) why this system fundamentally cannot work and will be abused. If you can't see that, then I'm sorry, but I'm going to have to turn you away from the polling station, because you're clearly unequipped to vote.