Newsbrief: Twitch has posted a blog clarifying its formerly "unustainable and unclear" policy around adult-oriented game streams on Twitch, formally banning streams of games granted an Adults Only rating by the ESRB (or equivalent ratings by other organizations.)
"Generally, if the US version is rated for Adults Only (18+) or has an equivalent rating in your territory, you should not broadcast that game on Twitch," the blog post reads.
This answers some questions but opens others -- for example, many adult-oriented games don't bother with ESRB ratings; there may be ratings discrepancies in other territories (Japan, Germany, and Australia each maintain their own ratings systems and are noted to be harsher on violence than the U.S., for example.)
Still, this serves as a basic guide and policy and will doubtlessly be clarified in the future. For now, Twitch points users to the ESRB list of AO-rated games as ones to avoid.