That's what's hard to gage. Likely due to what we consider nude on a woman isn't on a man. A shirtless man is completely acceptable, but a woman isn't. The discrepancy is cultural and the culture is discriminatory. This isn't going to change anytime soon.

The sensible thing would be to accept that what we consider appropriate for a man isn't for a woman. But this will always be pointed out as sexist. You could ban all nudity except arms, neck and knees down. But this will be criticised as excessive.

You could also allow all nudity as long as the context isn't sexual. However, twitch has a large market for sexual content particularly from people who may be limited from accessing that content like minors. So theres a large finical incentive for anyone to try to exploit allowed nudity.

Twitch likely want and encourage some of this content, as it drives engagement and thus profit. However, they want to minimise the legal/moral/reputation issues associated with delivering this content.