Features a bossrush, a single-life bossrush and three *unlockable* costumes that can be gotten by doing the bossrush. Honestly really surprised. Fantastic addition!
EDIT: all info here: https://blog.activision.com/sekiro/2020-07/Sekiro-Shadows-Die-Twice-Free-Update
EDIT: all info here: https://blog.activision.com/sekiro/2020-07/Sekiro-Shadows-Die-Twice-Free-Update