So who writes the literacy test? Should it require the degree of literacy required by a lawyer or a professional scholar of political science (limit franchise only to the best educated)? Should it test for correct attitudes (we wouldn’t want people who are homophobic or racist voting, would we?). Whose view of American history should we test (if you don’t realize that Barack Obama saved America, can you be trusted to vote).
In practice, America has 100% literacy. It should be noted, that literacy was never a required to die fighting for America. Literacy isn’t a requirement to pay taxes.