Okay, so we know that whenever someone was sorted into Slytherin at the beginning of the series they were booed, which first of all, dick move guys, some of you are like 17 and you are literally jeering at terrified 11 year olds. Come on douchebags, that is the worst way to treat them.
I like to think that after the war the booing stopped, but nobody cheered. While every other student would walk to their house table with pride and applause, young Slytherins walked to their future in silence. And maybe there was a few years in which they thought they didn’t deserve the applause, because we have to remember that during the final battle there was call to lock the Slytherins in their common room. Personally I think some totally snuck out to help, I refuse to actually believe that the most cunning, dedicated and savvy kids in that school stayed where they were told and let the people that they cared about get hurt without doing a damn thing. Just no. But anyway, I think there was a time when the kids of the Slytherin walked to their table without the cheers that everyone else heard.
But Slytherin is a proud house, and they always will be. So they sat through a few years without clapping at all because even the snake can pay penance when they believe it is due. Until one year the last witnesses of the Battle of Hogwarts are in their final year, this is their last sorting and whatever they leave is their legacy. And that year, when the first new Slytherin is sorted they cheer, as loudly and proudly as they can, and nobody else claps but they don’t need to because that year every Slytherin gets the loudest congratulations the Great Hall has heard in a long time.
And it continues like that for a while, the other houses don’t cheer for each new snake but Slytherin cheers for every single new student, regardless of their house. But then one year, like happens oh so often, two first years make friends on the train only to be sorted into different houses, the first one a Hufflepuff, and they watch their new friend be sorted into Slytherin and and cheer as loudly as they can because Hufflepuff loyalty right there. They cheer for their friend who whispered the secret of not wanting to be a Slytherin because even after all these years stereotypes are a bitch to kick, especially from the house who suffered under them the most. And slowly everyone joins in, just like they’ve always done for each student who wasn’t a Slytherin and by the end of that sorting every single student is applauded by everyone, no matter their house. The war is long over, and the Slytherin’s final battle is done.