After the battle in Kamino Ward, Rather playing like in canon, All for One was the one who won. He defeated all might by killing him gruesomely on live television. He also defeated all the Pro Heroes, completely becoming the demon lord he self-proclaimed to be

All for One😈: "The world is finally mine, Zen Shigaraki!" He said boastfully

And now, around a year, the world is an apocalyptic anarchy where everyone does what they want and uses their quirks to kill whatever gets in their way, danger if you find anyone if you are a suspect, nobody will hesitate to brutally murder you

Most of Class 1-A had fled away separated out of fear, or training, Izuku went in hiding...to train One For All under the radar not keeping contact with anyone, everyone was alone, trying to survive this messed up world