A project is dead when the source code is unavailable.
A project is dead when it is otherwise impossible to work on.
I would really only say these are fulfilled when all knowledge of the project is gone, but the internet makes this seemingly almost impossible. There are probably many projects from decades ago that have no surviving object code, source code, or documentation.
I wouldn't say older projects like ITS or Symbolics Genera are dead.
Of course, projects aren't living things, so they can almost always be 'revived'. You could say BeOS was dead, but then look at Haiku.