After hundreds of hours in the game, now that the devs finally fixed the civics bug, I am finally getting to know the Civic choices in the game. In my current game quite early in the Ancient I somehow managed to unlock a Civic called cultural eradication that COMPLETELY BLOCKS foreign cultural influence from your territories and keeps your culture 100% pure! WOW it's so strong! It's way deep in the Civic tree and I have no clue what triggered it as I didn't do anything special in this game. I kind of want to know the trigger though because it is so powerful and I want to repeat in my future games so if anyone else has run into this amazing Civic please let me know

You mean this?

YES that one! First time I ever saw it. Must be related to conquering a foreign city but that happens in all of my games and it never triggered before. It's either also chance based or there's some other condition I'm missing...