Is argument from ignorance always fallacious?

Hi, Artoria! 

Is arguing that a statement is false because it has not been proven true (or vice versa) always fallacious? Yes, always. Argument from ignorance just is the fallacy committed by arguing in that way. If an argument is not fallacious, ipso facto it could not be an argument from ignorance. 

Is arguing that a statement for which there is no proof is false (or vice versa) always fallacious? No, because it could be that when you are making the argument you are also including a crucial  premise that says that if the statement were true it would have been proven. This is why it is not fallacious to assert that unicorns never existed, to speak to Mr. Wednesday's answer. For we assert the statement not merely under the assumption that we have no paleontological proof, but that we would have paleontological proof if unicorns ever existed.

When the crucial premise is implicit, the illusion is cast that what you have is a non fallacious argument from ignorance. But such arguments are mythical creatures. What you have rather is not a non fallacious argument from ignorance, but a Modus Ponens with a tacit premise (or some other kind of argument.)

Thank you, Artotria

From, Kaiden 

argument from ignorance

Not always. You have to judge how reasonable the claim is, how much effort has been put into finding evidence, and how feasible it might be to find evidence. For instance, humans have been digging up and identifying fossils for hundreds of years, and we haven't found any unicorn skeletons. It is pretty reasonable to assert that unicorns never existed.