They added a feature where it can reference content from any conversation you've had with it in the past. It's not clear how this works - probably some kind of vector search?

Prior to that change "memory" was a tool call: https://simonwillison.net/2024/Oct/15/chatgpt-horoscopes/#ho...

they updated it opaquely - we no longer know anything about how it works https://www.latent.space/p/clippy-v-anton