According to this random website, the letter was opened in 1968 on the actual event of Gagarin's death and declassified in 2011 to celebrate the 50th anniversary of his flight.
The translation of the Gagarin’s letter in this article, which explains that it’s “translated by Yves Gauthier … author of a magnificent biography in French on Youri Gagarine” reads better than the translation in TFA, in my opinion
https://www.space-collectibles.com/blog/youri-gagarin-s-lett...