There are mentions about an old text. But no book or scroll or, suppose, clay tablet containing this text has ever been found. Or we can add that the text existed in different versions, in different languages. So that is why I would say, the text has been found in no form.
It has never been found in any form. If you say it has been found in no form, you have started the sentence saying that it has been found, which is the opposite of what you mean, and the reader has to undo that when he gets to "no".