Hello. I would see "forbidden knowledge" as information that someone on high deems harmful. I know that sounds "like a broken record" but let me explain. In religious contexts and literature, what is forbidden knowledge? It always would turn out to be something that could bring harm to a ruler's reign, endanger faith, or actually bring enlightenment, knowledge not ultimate realization, to peoples' minds. I would think that the term was made long ago to bring a sense of danger to information that would bring harm to a person's reputation or power. As with most of human history's rulers, they fear the loss of the only real thing they have left to lose: power. So the idea of "forbidden knowledge" was formed because it must of been so obvious that the ruler was hiding unsavory facts that the people beneath them created the legend. In summary of the above, I believe the idea of "forbidden knowledge" was made as a name for things that the ruler of an area or religious official did not want perpetuated, so they preached that those things were strictly forbidden.
Now onto your second question. I think that forbidden knowledge is really an opinion. For it to be called forbidden, someone must think that it would either cause harm or bring about something that they deem undesirable. Say that someone knows about a book, locked away somewhere, that if read would cause armageddon. That person might deem the contents "forbidden". In the other case, if a book were to bring about a global truth that God did not exist, then a Christian might once again deem that knowledge forbidden. So might a church or a religion collective in itself. So the nature of "forbidden knowledge", sorry for all the quotation, to me would most likely be someone's opinion regarding dangerous information. But what would play a key role is, who is the judge of that knowledge and who/what is the danger directed at? Many conspiracies about the government, church, and cults could fall into the categories above that might hide information about something to either protect themselves or us from a "danger" that they think would be harmful. Thank you for reading and brava on the excellent question. Good bye
P.S- Some of the views expressed or alluded to above are not my own and I might not agree with everything stated in the above text. Good bye.