This is straight from the current Content Guidelines:

“We don’t allow porn or sexually exploitative content on Substack, including any depictions of sexual acts for the sole purpose of sexual gratification. We do allow depictions of nudity for artistic, journalistic, or related purposes, as well as erotic literature, however, we have a strict no nudity policy for profile images. We may hide or remove explicit content from Substack’s discovery features, including search and on substack.com.”