The mysterious announcement is still hidden inside the game.
Recently, Eric "ConcernedApe" Barone gave an interview to IGN. The interviewer recalled his words: several years ago, the Stardew Valley developer mentioned that there is still a secret in the game.
Fans of the game still have not been able to find it, but the value of this secret is no longer particularly great:
Yes, [in Stardew Valley] there is still one secret that no one has discovered. I think no one will find it either. Maybe I will reveal it myself someday, but the problem with this secret is that... It is essentially a hidden message in the game that reveals something that is no longer even relevant now.
The creator of Stardew Valley continued and explained that it was a secret message-announcement, but later the plans changed and it lost its relevance:
It referred to one thing — should I reveal this or not? All right, I will only say that it was a secret message that effectively announced something, but then I changed my plans, and that message is no longer true, although it still remains in the game. And I just do not know whether anyone will ever find it, because it is hidden extremely well.