Well, 'itchy fanfiction' is a rather odd term. It might imply fanfiction that has a certain quality that makes you react in an 'itchy' way. Maybe it's because the writing style is so different from what you're used to. For example, it could use very unique or experimental language that gives you this itchy feeling like you're not quite sure how to take it all in.
I'm not entirely sure what 'itchy fanfiction' specifically means. It could potentially be fanfiction that has a particular theme or motif that is somewhat irritating or itchy in a metaphorical sense. For instance, if it repeatedly uses a trope that you don't like or if the characters are constantly in a state of annoyance or discomfort, it might be called itchy fanfiction.
Itchy fanfiction could refer to fan - created stories that might make the reader feel a sense of unease or 'itchiness' perhaps due to their strange or off - putting plotlines. It could also be that the stories are so engaging in a way that they 'itch' at your mind, making you want to keep reading to find out what happens next.