A (very basic, incomplete) timeline of the history of fanfiction
Got into fan studies like a month ago, and learning about how fanfiction has changed is definitely my favorite part. I should note that there were works you could classify as fanfiction as early as the eighteenth century, but I'm more interested in the evolution of modern fanfiction.
Beginning of fanfiction as we know it
1960s - 1970s - Women writing Star Trek fanzines basically invent fanfiction, and also the idea of having sex, which no one had thought to do before. Everyone say thank you women. The fandom term "slash" originates from early Kirk/Spock fanfiction. (Kirk SLASH Spock.)
1981 - Several women (because yes, they were also women) receive cease and desist letters from Maureen Garrett, director of the Official Star Wars Fan Club saying that "X-rated" fanfiction is strictly prohibited and that "the word has come down from George Lucas, himself, that STAR WARS pornography is unquestionably unacceptable." This managed to mostly deter people from writing Star Wars smut (until the Internet), an example of what Mel Stanfill calls successful "domestication" of a fanbase.
1990s - early 2000s - Fanfiction moves online. It is no longer necessary (or preferable) to create fanzines that are sent out to a mailing list - or traded at cons. Obviously people still make physical copies of zines and are still attending cons, but the internet makes available a plethora of free fan-made content. It also becomes harder (although as we shall see, not impossible) for companies to control what types of fanfiction get passed around.
Anne Rice and fair use (and why companies can basically do whatever they want)
2000 - Anne Rice, author of Interview With The Vampire, posts a message on her website stating that she disallows all fanfiction of her works. Several fanfiction writers receive cease and desist letters. The following year, fanfiction.net scrubs all fanfiction of her works from their site.
What Rice did would probably not have stood up to legal scrutiny, but that doesn't really matter. Fair use doesn't really mean shit because companies FREQUENTLY use language that exceeds their actual legal authority as a scare tactic. People don't know what constitutes fair use and even if they did, sites like YouTube (or like fanfiction.net) tend to side with the companies and IP owners. That means that a lot of what gets taken down is fair use but is stuff the IP owner finds objectionable, a la 1980s Lucasfilm. (See Mel Stanfill's Exploiting Fandom, chap. 4 for more about this)
Early mainstreaming of fanfiction: "And when everyone's a nerd...no one will be."
2006 - The infamous Harry Potter fanfiction "My Immortal" is posted on fanfiction.net. I originally thought about putting this here to be funny and cause you kind of have to, but I actually think the widespread ridicule it received is probably indicative of the fact that fanfiction was still seen as something weird and niche. (Yes, I'm aware this is not the only reason it's funny.)
2008 - Iron Man, the first movie of what would become the Marvel Cinematic Universe, hits theaters. I've seen people cite the MCU as the sole reason for the mainstreaming of nerd/geek culture. I think I have to agree, and with the mainstreaming of nerd/geek culture comes the mainstreaming of fanfiction.
2011 - Fifty Shades of Grey is published. The author, E. L. James, is open about the fact that her story started as Twilight fanfiction. This is one of the first (THE first?) books with mainstream success that is essentially published fanfiction.
Fanfiction goes mainstream?
2015ish to present: Is it weird to read or write fanfiction anymore? It doesn't really feel like it. The transformative works website Archive of Our Own has seen a steady increase in the number of visitors to the site every year since the 2010s. (And a lot of people, not just the weirdos, at least have some idea of what AO3 is.) It is no longer as taboo to turn your fanfiction into a published book. Which is remarkable, because as late as 2019, Mel Stanfill (to whom I owe my life) talks about how the ways in which women engage in fandom (fanfiction) are not profitable or career-advancing like the stuff male fans do (video game modding, amateur filmmaking). To me, it looks like that has changed. The fanfiction-turned-books that I've seen get the most snide laughs are After by Anna Todd (2014) and The Love Hypothesis by Ali Hazelwood (2021). But secure the bag regardless, ladies. We love to see a girlboss winning.
Further Reading (because obviously you're as invested as I am):
Fake Geek Girls: Fandom, Gender, and the Convergence Culture Industry by Suzanne Scott
Exploiting Fandom: How the Media Industry Seeks to Manipulate Fans by Mel Stanfill
Enterprising Women: Television Fandom and the Creation of Popular Myth by Camille Bacon-Smith
The Fan Fiction Studies Reader ed. Karen Hellokson and Kristina Busse
Fan Fiction and Fan Communities in the Age of the Internet ed. Karen Hellokson and Kristina Busse