No, it's not appropriate. Deku is a character with his own depth, goals, and story within the MHA universe. Fanfic should aim to explore his hero journey, his relationships in terms of mutual respect and support, not create content that focuses on inappropriate or disrespectful ideas like this.
I don't really have a list of specific 'mha deku x boobs fanfic' as it might lead to some inappropriate or objectifying content. Instead, if you like Deku, you could search for fanfics that focus on his growth as a hero, his friendships, and his challenges. For example, there are many fics that explore his relationship with All Might or his classmates in a positive and meaningful way.
You can find 'izuku support course fanfiction' on websites like Archive of Our Own (AO3). It has a large collection of fanfictions from various fandoms, and you're likely to find many related to Izuku in the support course there. Fanfiction.net is also a good place to look.
Well, there's a fanfic called 'The Support Course Hero: Izuku'. In this story, Izuku uses his knowledge from the support course to outwit villains in unexpected ways. It's really interesting as it gives a new perspective on his character. Also, 'Izuku's Support Course Journey' is quite popular. It details his relationships with other support course students and how they all contribute to the hero world together.
In most of such fanfictions, they might create evidence like hidden past connections. For example, AFO could be secretly involved in Izuku Midoriya's (Deku) family history. Maybe he had a relationship with Deku's mother long ago, which led to Deku's birth. But it's all fictional speculation within the fanfiction world.
An AU fanfic is a type of fan fiction where the story takes place in an alternate universe with different settings or circumstances than the original work.
An au fanfic is a type of fan-written story that takes the characters or setting of an existing work and places them in an alternate universe or situation that differs from the original.
Also, Uraraka is always there for Deku when he has self - doubts. She reminds him of how far he has come and how strong he is. In turn, Deku encourages Uraraka to aim higher and not be afraid to take risks in using her quirk in more creative ways, which is crucial for her growth as a hero.
One way is to have an open and honest conversation with the host family during the interview process. Au pairs can ask about the family's expectations and also share their own. And during the stay, if something is wrong, don't be afraid to speak up or seek help from the au pair agency. Au pairs should also trust their instincts. If something seems off during the initial contact with the family, it might be a sign to look for another opportunity.