It say 6 nature manipulation chakra not Nature transformation so what are you trying to get at. Also why hasn't my question been answered. What is the 6th?
Why hasn't anyone else use Yin-Yang release besides Juubi jinchuriki/RSM users?
What do you not understand? Yin release is not nature transformation nor Yang. They are elements. You need to combine them to get the nature transformation.
Also it's because they are Kekkei Genkai and they
have been used you just haven't seen it. Chakra rods are a good example, used by Nagato but because he had Rinnegan. Same with Madara.
Yin element and Yang element can be used to create a jutsu, but it won't be nature transformation. However, when you combine them you get nature transformation. But, you can't do it without the KKG.
Also I want to take back what I said, Yin-Yang is not even an element, it's just a KKG and nature transformation.
Nature transformation =/= element. Wood, Lava and Dust are all nature transformations yet none are an actual element as Yamato stated. The 6 nature transformations do not represent all elements. There are actually more than that.