yeah hikikomori is a really interesting phenomenon because of the way japanese society works. these people (usually guys in their 20s+) completely shut themselves away in a room and NEVER leave. ever. the thing that makes it japanese is that they're extremely private and shame is a HUGE part of their culture, so instead of their parents going in and dragging them out like most americans would usually do, their parents usually pretend it's not happening and usually assist in making it worse. like they'll bring them food or books or literally build walls up around them in the house so nobody knows they're in there because the parents are embarrassed of them and don't want anyone to know. there's some documentaries on youtube about it but they're all kinda old... it's getting more and more common with the rise of the internet.
most of the time it's caused by japan's really ridiculous societal standards. like man I feel sorry for so many of these kids. school life is vigorous, many kids going to school 7 days a week, studying for hours and hours (leaving school and then going to study at cram school until 10pm), the expectation of working 60+ hours a week when you finally get out of college at a really depressing or boring job that barely pays anything... also bullying is a HUGE issue in japan because if you're considered different, other kids will target you really bad because japan is a society that values group mentality over individuality. like in america we don't really stress that, but in japan hoooo boy it's extremely pushed that your value of a person comes from how you contribute to society and groups of people, not who you are as an individual. it's so much pressure that an increasing number of people just can't handle it and shut themselves away completely from outside life. in a way I can't blame them because I could never handle all these societal rules holy fuck. there's a reason why japan is like #2 in the world for suicides, especially because they don't take mental healthcare seriously at all. again with the group mentality... it doesn't matter if you're having issues, what's more important is how you contribute to society, so "just suck it up and deal with it" is their view on that. even kids in my classes that have special needs or have obvious mental illnesses... their parents are often so ashamed of it that they'll refuse to put them in a special needs program or get them help. and japanese schools can't force kids to go to psychologists or being in special groups, so the poor kid just suffers...
I think in her fic, she could definitely pull from ideas of hikikomori because it's definitely agoraphobia. the only issue is most hikikomori do it because they can't handle society and the expectations it puts on them as well as seeeerious and horrible bullying, not necessarily being traumatized by something specific. BUT! I mean there really isn't any "rules" to it of course. it would be awesome if she did research and looked at documentaries and pictures, because it's super fascinating to me to see where they're actually living. I feel like midoriya's mom and his friends would eventually yank him out of it before it got to be a lifetime thing like hikikomori typically is, but that would be super rad if he started showing serious signs of it!!