[currymj]

a lot of this was legally enforced until the 1970s. see e.g. the Racial Integrity Act in Virginia for one example. the state had a legally-codified definition of race ("one-drop rule") and put a lot of bureaucratic effort into keeping records of everyone's race, so that they could enforce segregation and bans on interracial marriage.

if Reconstruction had succeeded and Jim Crow had never happened, who knows what the salience of this would be today?

[bluGill]

Not everywhere though. Northern states didn't have nearly as much, but even in middle of nowhere small towns where no blacks even live people still know about it.