boolean. if true, indicates that the information contained within the object (content, etc) is considered "sensitive", in the sense that viewer discretion may be advised, e.g. "NSFW" material.
when encountered, usually the assumption is that the object's content will be hidden or collapsed behind a warning of some sort. in mastodon, the `summary` will be used as the text of the warning, and the `content` will be hidden below it; furthermore, anything in `attachment` will be hidden behind a sensitive content click-through instead of being directly previewable.