You might be surprised to learn that soy fiber is not a traditional fiber. It is a highly processed, modern invention similar to rayon.
According to The Whole Soy Story, Henry Ford was a great soy innovator and developed a plastic from it. He made an appearance in 1941 wearing a soybean fiber suit, but it "was itchy when dry, smelled like a wet dog when damp, and was so prone to ripping that he could not bend over or cross his legs."
Modern fibers and soy yarns are more refined. They still build on the idea of making plastic strands that simulate fibers.
There are some books that specifically mention the use of soy as a fiber and use it in some patterns such as No Sheep for You by Amy R. Singer.
Soy fabric is wrinkle resistant with some stretch. The natural color is soft ivory but the fibers can be dyed using acid dyes. They should be hand washed.
Some people have a soy allergy and need to avoid contact with it.
Fibers are created from soy through a chemical process similar to rayon and bamboo fiber production. It's a byproduct of tofu manufacturing. The liquefied protein is extruded into long strands of fiber.
I don't like that soy processing involves harsh chemicals. It's definitely not green or environmentally conscious. Actually, soybean farmers are responsible for a large portion of the destruction of the Amazon rain forest and large amounts of pesticide contamination.