E.g., in general it is not acceptable to say that the president of your own country is a lazy idiot even if you think so. In many countries it may not allowed to call someone a convicted thief, even if he is. May be the whole affair has passed for many years and the file is no longer accessible. In these cases law regulates the right of free speech. There is also a demand to forget - and to forgive, for the peace of a community.
Taboos are a way how community identify themselves.
Braking the taboo usually means to take a stand outside that community. Taking a deviating point of view marks the modern democratic societies as historical cuts - usually as complex new juridical and ideological arrangements. But as truths is not a direct derivative of true sentences, but also of many wrong or even dangerous thoughts, a reservoir for free thinking had to be made. But we should note that there grew new taboos in this scientifically enhanced societies. Sometimes this became visible - as in the cases of Giordano Bruno, Galileo Galilei, Wilhelm Reich or Timothy Leary.