I'm not sure specifically about a 'Linda Byler' in 'Fantastic Fiction'. There could be many characters and stories within that realm. It might be a relatively unknown or a very specific reference that I'm not familiar with.
Fanfics basically involve fans creating their own narratives based on the elements of something they love. It can be anything from imagining new adventures for characters to exploring alternate storylines. They're a way for fans to express their creativity and passion for the source material.
In BNHA fanfics, there are often ones that expand on the backstories of minor characters. This gives more depth to the overall BNHA universe. In AssClass fanfics, some might focus on the challenges the students face outside of school while still having Koro - sensei's influence. 'The World Beyond School' is a fanfic that does this well, showing how the students apply what they've learned in class to real - life situations.
It's hard to say they are widely popular. They are likely only popular within very specific, niche communities that are interested in the particular theme related to a cockring. Most mainstream audiences would not be into this type of fanfic.
First, you need a clear idea or concept for your fanfic. Then, decide on the characters and their development. Start writing with an engaging beginning and build a compelling plot.