Taboos are issues which, according to community standards are not to be discussed. Some social/cultural taboos appear almost universal, others arbitrary and specific to particular cultures. In open politics, taboo issues are usually labeled as out of scope, since they are not addressed as a public policy issue by other mass media outlets. In an open society, e.g. one with freedom of speech there are not many actual taboo issues, but rather ones that are controversial or highly sensitive.
An example issue: how much is the public willing to harass and abuse a person falsely accused of being a child pornographer?