Deku, who would eventually rise to become the greatest hero in the world despite starting out as a Quirkless teen, was first introduced to viewers when the first episode of My Hero Academia aired. They were also made aware of his primary interest: keeping notebooks. Deku's obsessive passion led him to compile 13 books before the events of Episode 1 that are filled with knowledge about heroes and their quirks. To be prepared in case he saw a hero in action, he always carried them with him. When Bakugou burned and threw one away, he was upset and promised All Might that the one he signed would become a family heirloom. These notebooks were established early on as a key element to Deku as a character, and many fans expected them to aid his journey to become a hero.
Deku's notebooks, however, have been abandoned as the series has gone on. They are merely a gag about his "hero fanboy" behavior. They only show up when Deku adopts this attitude, at which point he will mutter endlessly and may take out a notebook. The only other time they made an appearance in the anime was for the joint training with Class B in Season 5. Many people have written and read fanfiction in which Deku's love of analysis led him down a different path because this element didn't go as fans initially anticipated.
Why Are Deku's Notes Important?
Story elements, characters and anything else introduced in the first few chapters of manga or episodes of anime often wind up being crucial to the story as it continues. For example, both Bakugou and All Might have been important characters throughout the story's progression and have big moments yet to be adapted from the manga. Deku's notes are made just as important in the early stages of the series. They're the first part of him that truly got All Might's attention, and the information he's compiled in them has regularly assisted him in battle. Even his friends aren't saved from his obsessive writings, but his suggestions have since become powerful hero moves.
The fact that a piece of the main character, which was highly emphasized at the start of the series, has become just part of a gag in the late stages of the story is an odd move. The notebooks haven't gone away, but their treatment has slowly changed as the series has progressed. They've gone from Deku's biggest strength to a silly character trait. Many will argue that this is for a good reason. Deku's focus has shifted from analyzing others to bettering himself, as he needs to increase his body's strength and master the many Quirks within One For All. As a result, his learning has moved from analytical to practical, giving him less time or reason to take notes. However, there was one moment that would have been very different had he used this skill.
Deku's Notes Might Have Played a Significant Part
The opportunity to bring the notebooks back and reveal shocking information along the way was right there for My Hero to seize. A notebook containing all the data on the previous One for All users, including their names, Quirks, and death records, is put together in Episode 113. Deku's visions of the previous users and his cursory knowledge from the series could have served as the starting points for this beat in the narrative, followed by his researching the information from the internet and other sources. He could have independently arrived at each and every conclusion this episode offered, then brought his findings back to All Might and Bakugou.
Instead, All Might did all the work for him. This implies that Deku, the genius note-taker, didn't think to research the people and Quirks residing inside him. Whether intentional or not, this may feel out of character for him. Deku, who took notes on random heroes on the street all the time, should have felt compelled to write down everything he could about the powers he was going to get and where they came from. One might argue that All Might had more resources and time to compile these notes, but it can be disappointing when a character is simply handed information like this. Deku had the skills and the tools to do it himself, and that could have led to more impactful moments. Whether Horikoshi did this intentionally or not, it is disappointing that Deku's notes weren't as important as the start of the series made them out to be.