All Might would be furious. He would see it as his failure to protect Izuku. He would probably mobilize all the resources he has, both in his hero form and his connections in the hero world, to rescue Izuku. And he would be blaming himself all the while for not being there when Izuku was kidnapped.
All Might would probably be very supportive. He's always been about the spirit of heroism, not gender. He'd encourage Izuku (as a girl) to keep training hard. Bakugo might initially be shocked and then try to act all tough, but deep down he might be a bit confused about his feelings towards her. Ochaco would be excited to have another girl around and might become even closer friends with Izuku.
Some in 1A might be in awe. For example, if Izuku has a particularly powerful or unique quirk in fanfiction, characters like Mineta might be jealous. He often shows envy towards those with strong abilities.
Izuku might be shocked at first. After all, he has grown up without his father. But then he could be filled with a mix of emotions like joy, confusion, and anger all at once.
Bakugou might be initially shocked. He's used to Izuku calling him 'kacchan' for so long that it's become a normal part of their relationship. So, when Izuku stops, it could throw him off balance.
Bakugo might initially be in denial. He's always had a complex relationship with Izuku, and seeing him in such a state could shatter his world view. He might become more withdrawn or, on the contrary, be spurred into a violent rage against whoever was responsible.
In 'Class 1 - A's AUs Adventure', they not only watch different AUs but also get to interact with their alternate selves in a very unique way. The story is full of humor and some really cool fight scenes that are different from the original series because of the AU elements.
You can start by looking on Archive of Our Own (AO3). It has a large collection of fanfictions, and if you search for 'bnha class 1a watches aus', you're likely to find many relevant stories. AO3 is known for its wide variety of fan - created content.
One common theme is the struggle with mental health. The suicidal reader character often represents someone who is in a very dark place emotionally. Another theme can be the discovery of hope through the encounter with the sanses aus characters. They might offer support or a different perspective that starts to change the reader's outlook.
Well, 'abused reader' might refer to a reader character in a story who is being mistreated. 'Jumping aus' could potentially be a misspelling or some sort of shorthand for 'jumping across' different AUs (Alternate Universes) in Undertale fanfiction. Undertale has a huge fanfiction community with various takes on different universes, and this could be about a plot where a reader - insert (the 'abused reader') is moving between these different AUs in some way.