Is political correctness going too far? - a liberal perspective

Am I the only bleeding-heart holier-than-thou liberal who is starting to get a bit ticked off with the witch-hunt trajectory that political correctness policing seems to be taking these days, where transgressions against modern norms and values are being revisited on people forty years later.
As yet another Virginia politician gets hauled over the coals for having dressed up as a rapper at a fancy-dress party back in 1980, I am starting to wonder whether we maybe need some kind of statute of limitations on these things. The world was a very different place back then, and people just did things like that. Maybe that doesn't excuse them completely, but how was the teenaged Mark Herring - yes, he was a teenager! - to know then that one day it wouldn't be considered acceptable (I'm assuming that the Virginia Attorney-General wouldn't do the same thing today).
We are not talking here about someone who regularly attended KKK events, or someone who has a long-standing history of racism and deliberate discrimination. And I am not suggesting that Hitler be excused his actions on the grounds that it was an acceptable practice back in the 30s and 40s. I'm just saying that some sense of proportion is called for, and that a competent and reasonable politician (which is what I am assuming Mr. Herring is - I don't know him from Adam) should not have have his career destroyed for a one-off youthful indiscretion, which he was probably not even aware of at the time.
I may be pushing my luck here, but I'm not that sure that it should be verboten even today for people to dress up as something they're not - isn't that the point of a fancy-dress party? How is it so different for someone to dress up as a Disney princess than it is for them to appear as their favourite singer (who may or may not happen to be black)? As far as I can see, it's not even that it's disrespectful; it's more a form of homage. How is it putting putting black people - or indigenous people or Asian people or whatever - at any disadvantage?
Ah, my bleeding heart is clearly starting to congeal and calcify. But am I wrong? What am I missing?