I read all these threads about coins becoming officially dead. However, most of these threads are FUD. When does a coin become official dead? Is it when the developer stops developing? Is it when the coin stops trading on major exchanges?
You've actually answered yourself, nobody will officially inform you that their coin is dead; you're going to have to deduce that by your own observations and you stated teo(2) of them above. All dead coins have their developers abandoning project, a drastic fall in prices on the exchanges or it is no more listed on the exchanges.