Earlier quoted context omitted.
I'm not even American and yet where I'm from it's common knowledge that literacy tests were used to systematically disenfranchise black people from voting.
Right, and that was apparently constitutional. If so we can bring back the law, but this time to disenfranchise dumb voters.
Keep in mind that the word "dumb" gets thrown around in all sorts of situations and has no objective, easily-verifiable test for "dumbness". It's entirely plausible that merely "making a decision I disagree with" could be construed as "dumb". Imagine if, say, Biden and Fauci had decided to disenfranchise anyone who refused the COVID vaccine as "dumb", or, worse, Trump and RFK decided that taking those vaccines made you "dumb". See how easily this can be abused?
Even with that aside, the natural consequence of disenfranchising "dumb" people is that you want to fuck them over somehow. No thanks.