Twitch wants to be more transparent in its rules for nudity and clothing, to make sure you do not wear transparent clothing on camera.
In a nutshell, Twitch has got a lot more specific in what is allowed and what is not. Previous guidance largely amounted to 'wear what is appropriate for being in public'.
So the general rule remains the same - there should be no nudity. Not fully or partially. And absolutely no genitals or buttocks.
But there's also some new specifics. No outline of genitals should be seen even if covered. And they should be covered - the whole area "extending from your hips to the bottom of your pelvis and buttocks", Twitch has clarified. This means opaque material, no partially see-through stuff.
Breasts? "For those who present as women, we ask that you cover your nipples." What Twitch defines as "exposed underbust" is not permitted either, though "cleavage is unrestricted as long as these coverage requirements are met".
Cosplay and digital filters/avatars must follow all of the same rules.
There are no exceptions for educational purposes or nude art drawing.
There are exceptions to upper body requirements for those who are "actively breastfeeding", and those who are applying body art or if you are streaming at the beach (though those who present as women must still cover nipples).
So now you know.