I don't know specifically which rules the parent is referring to--their phrasing ("Post US Civil war, we encoded a set of rules") makes it sound like it is referring to the US as a whole, but I'm not sure if that is their intent. Some rules at the state level, before they were forbidden by the Voting Rights Act of 1965, included poll taxes and literacy tests. This is also where the term "grandfather clause" originates, as people whose grandfathers could vote (i.e. whites) were sometimes exempted from these requirements.
> evidence that they were intended
I will say that evidence of intent is not necessary or relevant to the claim. Subjectivity in the application and execution of the voting process--for instance, where to place polling stations--can end up disadvantaging minorities due to implicit/unconscious bias in administrators, even unintentionally.