Yandere Simulator follows the adventures of a high school girl on a quest to seduce the boy of her dreams. Unfortunately for her schoolmates, she's not above kidnapping and killing each of them to make sure she'll be the one to win his heart. The game features gratuitous violence, and a lot of teenage girls in their underwear, however, Twitch has not given a specific reason for the banning as of now.
The developer, known as YandereDev or Alex, has reached out to Twitch to inquire about the ban, but while Yandere, where girls fall for a boy so hard they threaten, harm, or kill any girl that may seem interested in him, is a fairly common theme in Japan, it's not surprising that Twitch wants to distance itself from a game that stars an adolescent murderer, even if it is all in good fun.
However, this exerpt from Twitch's Rules of Conduct make it very unlikely he'll get far:
"To protect the integrity of our community, as the provider of the service, we at Twitch reserve the right to suspend any account at any time for any conduct that we determine to be inappropriate or harmful. You are free to appeal your suspension, but we are not required to provide a reason for our decision."
Even if you can't watch a stream, you can still play the early Alpha of Yandere Simulator and keep tabs on its development at the developer's blog.