No, they're not irrelevant. It doesn't matter whether or not Democrats support slavery/segregation now, because no rational person in either party would want to bring either of those things back. What matters is how the parties stood on those issies when they existed. When they existed, slavery and segregation were both supported by Democrats and opposed by Republicans. Doesn't that ever give you pause, at least for a moment?
No more pause than Republicans being anti-gay rights in the 2012. . . . . . . . . before the pretty dramatic shift amongst the party towards gay marriage that seems to be getting more and more Republican
lawmakers behind it's cause by the day.
Granted, Republicans warmed up to gay marriage a lot quicker than Democrats to desegregation.