Technically, Luna Lovegood hasn't seen anyone die in the books directly. While she was present at many battles and dangerous situations, the deaths we see occur off-screen, like Fred Weasley or Dobby.
However, she has witnessed the aftermath of death. She was at the Battle of Hogwarts and would have seen the bodies of those who were killed. She also likely heard about the deaths of her friends, family, and other members of the wizarding world.
So, while she hasn't directly seen anyone die, she has undoubtedly been affected by death and loss throughout the series.