Forbidden knowledge is commonly not secret, rather a society or various institutions will use repressive mechanisms to either completely prevent the publication of information they find objectionable or dangerous (censorship), or failing that, to try to reduce the public's trust in such information (propaganda).
Jokes throughout history have been a powerful instrument to undermine state authority and the public truth associated with it.
Some form of public repression of facts or speculation not desirable to some people or even a majority of the population seems inevitable as societies need to create some common basis of facts to create a unified identity.
Western secular societies have reached the consensus through the late 19th and early 20th centuries that private organizations should not be allowed to engage in compulsory censorship, forcing people to obey their dictates.
For example, the separation of church and state in most Western societies mostly prevents religious organizations from repressing individuals based on their personal opinions and beliefs.