In this case the subject would have been defined because it would have come earlier in the conversation. Based on this sentence, you can presume the "it" they are discussing is a common (-en) word.
If you were translating this sentence from english to danish, and didn't have any other context, den or det would be acceptable.
True, but I think it's worth pointing out that if you were to say "It exists no more" to someone, they would most likely give you a strange look.
It's certainly useful to know archaic and unusual expressions, but I think the Danish here is everyday Danish -- please correct me if I'm wrong -- and so it should be translated into everyday English unless you have a good reason not to.