Twitch has made amendments to its Rules of Conduct which are outlined in a blog post and prevents users from streaming any game which the ESRB classifies as AO (Adult Only) or 18+ equivalent in Europe. AO is a rating which is rarely used in North America as retailers often refuse to sell those titles. This means games like GTA get the lower M for Mature rating (17+).
For those using PEGI and other global rating systems where 18+ certificate is actually used properly, Twitch has that covered too. Any game that Twitch deems to be AO will be banned from streams, so GTA V (18+ rated in Europe) will be fine, HATRED (which doesn’t actually have an ESRB rating at all), Leisure Suit Larry: Magna Cum Laude and Manhunt 2 (18+ rated) will be blocked.
The full list of banned titles can be seen by looking at the AO rated titles on Wikipedia.
Whether Twitch will enforce the rules is another matter as their previous rule changes on how much flesh should be seen is often ignored as can be seen in many thumbnails when browsing popular streams.
Perhaps YouTube Live and Hitbox.tv will pick up on the market that Twitch is leaving open but only time will tell.