From what I saw on the forums and internet, when the Sims 4 first launched there was a Memory Timeline which you could access through the Camera Controls. When you took a screenshot you could make it a memory and then it would show up on the individual Sims Memory Timeline. You can still tag screenshots as memories via the Screenshot Capture Manager but its just a mishmash of every screenshot you've taken. You can still tag your sims in those screenshots and write a little blurb on them, but its not the same as having them in the timeline.
