Board Thread:Rebirth: Applications/@comment-8914761-20141111004451/@comment-28861328-20141111202054

To answer your question, Sig, Ryun has everything to do with it. He was the head of Yamagakure's branch of the Uchiha Clan, as well as its founder. When it came to clan decisions, such as what history they should record, what techniques they should cultivate, and so forth, it was all his call. The Uchiha Clan in Yamagakure, culturally, are different from those still in Konohagakure. Most (not all, mind you) haven't fallen into the Curse of Hatred, the majority are strongly loyal to their families (where, as Void pointed out, the former Uchiha Clan in Konohagakure frequently murdered one another to achieve higher power) and their village, and many wish to forget the atrocity and horrors the main clan once took part in.

Izanagi was a technique that rewrote fate and was a symbol of the Uchiha's previous narcissism and lust for power. That, along with the fact that it blinds the Uchiha's Sharingan, would say to me, knowing how Ryun leads, that he wouldn't have chosen to record the information or keep records on it. His own children don't even know the ability. Izanami was a technique invented to stop the users of Izanagi and save them from themselves, so without Izanagi, Izanami isn't needed and wouldn't have been recorded either.

The Konohagakure branch may be another story entirely. Perhaps Sasuke kept records of those Uchiha forbidden techniques, but Ryun doesn't strike me as the kind who would keep them around.