What is the oldest written source in this country that mentions this right?
South Sudan
Article 22 of the 2011 South Sudan constitution protects the right to privacy: “The privacy of all persons shall be inviolable; no person shall be subjected to interference with his or her private life, family, home or correspondence, save in accordance with the law” (Constitute Project, “South Sudan 2011 rev. 2013”).
https://constituteproject.org/constitution/South_Sudan_2013?lang=en