Diaries, whether published or unpublished, are protected by copyright. The diarist holds the copyright to the content of the diary, even after their death. The copyright holder has control over reproducing and distributing excerpts from the diary. Diaries can be sold, but the purchaser does not hold the copyright. Most posthumous copyrights are held by the heirs of the diarist. If someone wants to publish a diary, they need to obtain permission from the copyright holder. Diaries that have entered the public domain can be freely published. Burning diaries is an option to prevent them from being read or published after death.
https://hughstephensblog.net/2023/07/28/copyright-and-the-inveterate-diarist/
https://hughstephensblog.net/2023/07/28/copyright-and-the-inveterate-diarist/