Nothing that is extremely important to understand the plot is outside of the game. Everything that is not in the game is extra knowledge, not required one.

As example, yes, the fact that 2B had to kill 9S exactly 48 times is an info that is not found in the game, but the general fact that she had to do it previously it's something that becomes very clear in the game with no need of extra info: the clenched fist during the cutscene right after the end of the prologue hints at the fact that she's angry at the fact that 9S's memories(in regards of what happened during the prologue) are gone, but the prologue itself taught us that this is not something extremely uncommon, yet 2B is very angry at it, even if she's the one saying emotions are prohibited...something clearly happened before the game events that we don't know about. This scene puts the player on the look out for more information about this, so along the game you tend to notice(thanks to small sentences and nuisances in their dialogues) how 2B's attitude towards 9S hides the fact that, despite ultimately failing at it, she tries to not get too close to 9S...at this point the suspicion that