The Japanese anime industry did not invent the concept of a fictional character who ends up in an exotic fantasy world to explore. There are iconic predecessors like The Wizard of Oz and The Chronicles of Narnia that deconstruct this concept decades earlier than anime. Still, this premise didn't become a proper subgenre until anime series like Sword Art Online started to air, which has helped isekai evolve into the massive subgenre that it is today. Sword Art Online may not be anime's first isekai story, but it's still a historical and significant milestone.
Source：https://www.cbr.com/sword-art-online-classic-anime-why/
