It was officially released on October 10, 2021.
In the full game, after beating 202X Mode for the first time, a new difficulty mode may be unlocked: 1980s Mode. This mode can only be accessed through a save file that was used to beat the game. Currently, the mode can be accessed directly from the title screen, before the main menu has even appeared.
In 1980s Mode, the main character is Ryoba Aishi, who is Ayano Aishi's mother, as well as the yandere girl featured in the Mysterious Tapes. Her appearance will resemble the Ayano from the save file that was used to start 1980s Mode.
This mode only has technology that was available in 1989, along with stricter school rules. There is no internet, smartphones, Info-chan, or anything else that did not exist in 1989. The smartphone is replaced with a notepad, and students that witness a murder call the police using the landline phone in the Faculty Room. There is a Yakuza in this mode, unlike in 202X Mode. There is an optional filter that makes the game look grainier and older, much like a movie on VHS, to help represent the fact that this mode takes place in the 1980s. Basement Tapes 1 to 9 are the epilogue of 1980s Mode.
The objective of this mode is to defeat 10 rivals, like in 202X Mode. However, the plot of 1980s Mode is not as elaborate as 202X Mode. For example, all the events between Jokichi Yudasei and the ten rivals are the same, but with different dialogue.
The endings for this mode vary, but are determined by the player's "grade", or rank of how they played, eliminated the rivals, and dealt with homicide and any cleanup. The S+ rank unlocks the canon ending, but it can only be found if certain events play out beforehand.
Differences to 202X Mode
- The background music was completely remixed/remade in a synthwave style to match the 80s style.
- Because smartphones did not exist in 1980s Mode, the pause screen was changed from a smartphone to a notepad. Instead of students calling the police on their phones, they will call the police on the landline phone located in the Faculty Room.
- All Student's Profile in the pause screen are loaded up at the start of the game, as you cannot take pictures with a phone.
- Ryoba Aishi's room is different from Ayano Aishi's room in 202X, and the video game console in her room is the "Mafukuru Entertainment System", which is a reference to the original Nintendo Entertainment System.
- Info-chan did not yet exist, meaning it is not possible to see the schemes' steps. However, all schemes are unlockable at the start of the game. However, some of the schemes must be unlocked by the serial killer at the town.
- Ryoba is not threatening to Kocho Shuyona in this mode. If she encounters him, Kocho talks to her in a friendly way while in his office and will not try to tase her if she is close to him. But he will tase Ryoba if he sees her carrying a weapon or a corpse in front of him.
- Some of the students that were previously removed, such as the Rainbow Students, now appear in 1980s Mode.
- Some of the electronics, such as the radio, are redesigned so they can fit the style of the 1980s.
- All Student Council members were male in 1980s, and they are functionally identical to the 202X Mode student council.
- The calendar screen now has "VHS aesthetic effects", and there's also a new menu called "stat" where the player can see their actions, such as Rival Elimination Methods, in the previous weeks.
- Ryoba will now be able to interact with the Yakuza at the town at night. However, the Yakuza will not speak with Ryoba unless she completed Togo Atatsuma's task.
- The exclusive persona in 1980s Mode is the Snitch persona, which replaces the Social Butterfly persona. The students with this persona will run to the faculty room and call the police on the landline phone if they discover a corpse or witness murder, and will stay there until the police arrive. This makes it impossible to kill them, as they will be surrounded by teachers.
- The Social Butterfly and Phone Addict personas do not appear in this mode, as those with said personas use their phones. Spiteful does not appear either because there are no bullies, which is most likely the reason Yui Rio and Haruto Yuto are Loners, even though Spiteful students approve of the death of a student whose reputation is low enough. Fragile doesn't appear and the reason is the same as Spiteful. Protective also doesn't appear because YandereDev said that the rivals have no unique traits nor obstacles, excluding Chigusa and Sonoko but their obstacles don't have this persona.
- The Journalist will be investigating Ryoba's suspicious behavior while walking in front of the school gate. If he sees Ryoba kill someone, being bloody, or carrying a weapon while standing too close to him, he will quickly run up to her and pepper spray her, resulting in a Game Over.
- The Sociology Classroom is in place of the Gaming Club in this mode.
- The Info Club has been changed into the Newspaper Club, which is now a fully-functioning club Ryoba can join.
- There is a grade you will receive at the end, depending on the way you played the game.
- There are now 10 delinquents instead of five, with half of them being female.
- The Saikou laptops aren't present at Meeting Room or the Student Council Room as laptops did not exist back then.
- The player has access to a list of debug commands at the Calendar screen which allow them to change the events of the previous weeks, such as how they eliminated their rivals.
- The player cannot take photos as they don't have smartphone with them. However, they can do so if they join the Photography Club as the old camera will be provided to them.
- Since wireless headsets didn't exist in 1989, the player has to give the suitor advice before the daily interaction for the matchmaking process, and learn the outcome afterwards.
- Sumire Saitozaki (Tutorial rival)
- Kaguya Wakaizumi (First rival)
- Moeko Rakuyona (Second rival)
- Honami Hodoshima (Third rival)
- Sumiko Tachibana (Fourth rival)
- Ritsuko Chikanari (Fifth rival)
- Ai Doruyashi (Sixth rival)
- Teiko Nabatasai (Seventh rival)
- Komako Funakoshi (Eighth rival)
- Chigusa Busujima (Ninth rival)
- Sonoko Sakanoue (Tenth and final rival)
The "rank" given at the end will depend on your "guilty points" and "innocent points".
- The police are called to school: +1 Guilty Point per visit.
- The police discover a corpse on School grounds: +1 Guilty Point per corpse.
- Student catches Ryoba doing something suspicious: +1 Guilty Point per student (Senpai noticing you in his aura does not count).
- Student accuses Ryoba of murder: +20 Guilty Points per student.
- The rival's death is judged to be a murder: +10 Guilty Points.
- The rival's death is considered to be an accident / she has gone missing: No Guilty Points gained.
- Befriend a student in school: +1 Innocent Point per student.
- The Rival befriended Ryoba and testifies she would never be a killer: +10 innocent points.
- Each reputation point is worth +1 innocent point, with a maximum of +100 reputation points available to Ryoba.
At the end of the game, if you have more guilty points than innocent points, you'll be found guilty of murder and earn Rank F. If you have at least 100 more innocent points than guilty points, you'll be found innocent, free of suspicion and earn Rank S. Anywhere between these two and you'll earn Rank A, B, C, or D.
As for Rank S+, Ryoba will hint at the beginning of each week how she wants her rival eliminated:
- Kaguya Wakaizumi: Sabotage her interactions with Senpai so he'll reject her love confession.
- Moeko Rakuyona: Burn her to death and make her death look like an accident.
- Honami Hodoshima: When she enters the library, push a bookshelf on her, crushing her to death. Her death will be considered an accident.
- Sumiko Tachibana: Poison her bento using a lethal poison. Her death will also be considered an accident.
- Ritsuko Chikanari: When she sunbathes, push her lounge chair into the school pool to drown her as she is sleeping (To make her sleep, you must poison her bento with a Sedative). Her death will be considered an accident.
- Ai Doruyashi: Have her electrocuted to death during her daily performance on stage in the school's gymnasium. Her death will be considered an accident.
- Teiko Nabatasai: Have her expelled from school. Framing her for possession of Narcotics will not be counted, as she will get arrested afterward.
- Komako Funakoshi: Gossip about her until her reputation reaches -100 or below. This will cause her to withdraw from Akademi. If her reputation reaches -150 or below, she'll kill herself, which does not give you S+.
- Chigusa Busujima: Matchmake her with her suitor, Etsuji Odaka.
- Sonoko Sakanoue: Befriend her and ask her to stay away from Senpai.
Following through with each specific elimination and earning at least 100 more innocent points than guilty points will earn you rank S+. It will also unlock a bonus cutscene at the end of the game as well as a timeline.
- Before the events of 1980s Mode, there was a murder at Akademi which prompted the school to force faculty members to undergo self-defense training before the player starts 1980s Mode. The victim was Sumire Saitozaki.
- The idea for 1980s Mode occurred when someone complained that they did not like Info-chan and wanted a mode without her.
- YandereDev would like help from volunteers to make period-correct outfits and hairstyles for the 1980s.
- This mode features a brand new student body with a different configuration of colors and accessories.
- The Info Club room is the Newspaper Club in this mode.
- The Newspaper Club also replaces the Gaming Club in this mode.
- The Journalist appears in front of the gates at school and in multiple cutscenes.
- Senpai's parents do not appear in this mode.
- The events of the 202x Mode are partially determined by the events of 1989, so there will be a "canon" way to eliminate each rival. If the player chooses to go with the canon elimination methods, then the Mysterious Tapes are proven true.
- This mode features the death of Sumire Saitozaki, also known as the Phantom Girl.
- She is killed in the tutorial to avoid a time paradox.
- This mode features the previous Delinquents, the removed rainbow students, the Study-kun boys and Witness-chan, who is now named Ui Tunesu. YandereDev said that if the framerate is more stable enough and there is more room for more students, then the rainbow students will be in 202X Mode, and if not, then they'll stay in this mode. 
- This mode is meant to be the most difficult mode. It may include the player getting a Game Over if a witness escapes, since in the Mysterious Tapes, the yandere of 1989 was arrested with no hard evidence.
- YandereDev has stated that he will attempt to give this mode the same vibe as "a peaceful countryside Japanese town in the late 80s".
- This mode is ten weeks long.
- The outcome of this mode will not affect the story.
- Genka Kunahito's mother, Mae Kunahito, appears in this mode.
- The overwhelming majority of the new music in 1980s Mode was created by one person, CameronF305. 
- While passing time, Ryoba looks at a polaroid of Jokichi, rather than a phone.
- Canonically, half the rivals were killed, and the other half were eliminated peacefully.
- In 1980s mode, absolutely everyone participates in small amounts of bullying instead of one specific group doing massive amounts of bullying.
- Since there aren't any bullies, students will not have their schedule changed if they have a low reputation.
- 1980s Mode is currently the only mode where Ayano does not appear as a playable character as she did not exist yet.
- Mission Mode is not available, since there was no one to send Ryoba to kill.
- Originally, there was going to be a rival for 1980s mode who was a shy and antisocial girl who had trouble communicating with others. She wanted to become a meteorologist after she graduated, so she would spend most of her time outdoors near meteorological equipment, studying the weather. During her week, there would be a lightning storm. While all the other students would go inside, she would stay outside in the rain next to her meteorological equipment to study the thunderstorm and gather data. There would have been an event where you would sabotage her equipment so it would overload and electrify her. Ryoba would frame this as a lightning strike, as you'd make it look like she was struck by lightning while studying the lightning storm. In the end, this concept was rejected because it would have taken too much time to set up a weather system for just one day.
- As of the May 15th, 2022 build, the week select screen now has a 3D interface when selecting a week to start off with.