there is nothing called officially in my opinion.
you can call a coin dead for many reasons for example:

• when the developer gives up on his own project and nobody else is interested in picking up where he left off.
• when the price goes to 1 satoshi and there is no buy orders and nobody is buying from the asks either.
• you can even go as far as calling a coin dead when it is getting dumped real hard down to 1 satoshi
• technically a coin is dead if there is no one mining it and there is no full node of it available any more.