I saw there is a rule said do not Creating comments that exceed the rating of the submission, does this also apply to descriptions? Or can we add some rule to prevent this?

For example, post #3530942 (warning: nsfw cub/rape content in description) is rating:s but the description is obviously nsfw.

I think the difference here is that an explicit comment on a safe post is generally creepy and unwelcome, but the contents of a description is usually commentary from the artist that specifically relates to the image.

Perhaps descriptions that exceed the rating of the submission should be hidden inside a collapsible section with an explicit warning in the title?

The description on that example post should be put into a section anyway purely because of how long it is.

Descriptions should generally not be used for any content other than that written by the artist. The corollary of that is that any content written by the artist is eligible for the description. (I guess unless it breaks one of the really bad rules that gets comments deleted rather than just slapped with the red text of shame, but you probably shouldn't upload works like that anyway.)