Technically True coin Death occurs when the Chain Freezes and coin transfer is no longer possible.
99% of the users have given up on it way before that point is reached.
I would add that unless the coin is active on at least one exchange with some amount of volume, its clearly heading for demise.
You also need at least one developer doing work, pushing updates on Github etc. Even if updates only happen a couple times a year, you need some sign that someone is still looking at the code and trying to make improvement.