Just today I came across a conversation of Twitter, on how fans are not able to separate fiction from reality, based on an ao3 fic and it's comments. There are in general a lot of positive sides in writing or reading fanfics, but they can also have negative implications on the (sexual) development of young people, if they are exposed to the wrong thing - and by how and where the stories are told - this can happen too easily.

I think y/n fiction are also rather normal and have been a part of pop culture for a really long time - just not in this form.

If someone is absolutely able to differentiate fiction from reality and those fictional stories stay within their clearly marked spaces, I don't have a problem with it. Sadly, I observed that this is not as common as I had previously assumed. Instead, kpop fandoms are drowning in their headcanon and are overwriting real people with their fictional shared story narratives, which is often a significant factor in hateful and toxic narratives against real people.