the thirty-year rule
noun /ðə ˌθɜːti ˈjɪə ruːl/,  /ðə ˌθɜːti ˈjɜː ruːl/
  /ðə ˌθɜːrti ˈjɪr ruːl/
[singular]- a British government rule which prevented certain official documents from being made public until a period of thirty years had passed. It was replaced when the Freedom of Information Act was passed in 2005.