This question already has an answer here:
What was the first story arc to explore a major superhero death?
To avoid broad interpretations:
- By superheroes I also mean the ones with or without superpowers just like Young Allies, Batman, Bucky Barnes, Rocketeer / etc.
- By story arc to explore I mean the ones that is the target of the history it self. Example: Death of Captain Mar-Vell, Superman Death, Barry Allen. NOT: uncle Ben nor Thomas Wayne and Martha Wayne.
- By death I mean also the ones that maybe alive in alternative universes or reborn after a good time in the refrigerator.