The main point is that if you are going to give a timescale, we usually use "died + timescale". It's not obligatory to use a timescale.
She died last week.
She is dead.
He died in 1942.
- What happened to your uncle?
- He's dead.
- I'm so sorry. When did he die?
- He died last week.
She died last week = common
She's been dead since last week = uncommon in normal everyday speech. It might be used by medical/police when they find a dead body and want to give an idea how long ago the death occurred.
It really depends on whether you simply want to refer to the person's physical state - a person is either alive or dead - or to make the act of dying a verb.
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