Plessy was overturned by the Civil Rights Act of 1964.
You posit the legally impossible, namely that a legislatrive act overturned a provision of the Constitution as interpreted by the U.S. Supreme Court.
Plessy (separate but equal) was reversed by Brown v. Board of Education, 347 U.S. 483, 495 (1954).
We conclude that in the field of public education the doctrine of "separate but equal" has no place. Separate educational facilities are inherently unequal."' Therefore, we hold that the plaintiffs and others similarly situated for whom the actions have been brought are, by reason of the segregation complained of, deprived of the equal protection of the laws guaranteed by the Fourteenth Amendment.
Yes, that didn't sound right to me either, but I didn't want to argue about that particular point.