Playing Indigo League is only legally defensible if you dump your own FireRed cartridge and apply a patch you created from publicly available patch data – but even then, it violates Nintendo’s terms of service.
Pokémon Indigo League is a fan-made game that recreates the first season of the iconic Pokémon anime as a playable experience. Unlike the original Pokémon Red , this game follows the specific journey of Ash Ketchum Key Game Features Anime-Driven Story : The game mirrors the events of the Pokémon: Indigo League