I think I found the memory system in TS4 - Memory Panel

Memories used to get saved per sim and back in the beginning you could also choose what emotion to attach to the memory. It was just a small boost, but came in handy for storytelling. I turned my very first screenshot of my first Sims 4 family moving in into a sad memory, because I imagined that my sim was sad to have to leave his old home.

After they changed the system I stopped using memories, because everything is too cluttered now.