Actually, I don't know if something changed in the meantime, but last I heard it was to be released around June 20th.
I usually like to play final versions of my games. I hate it when I play a game, I build my characters, and then a patch is released that nerfs my builds. All of a sudden those artifacts, that I've been hunting for throughout my game, become useless, while the items I've been selling from the start become invaluable. It has more to do with my past experiences, than it does with Darkest Dungeon in particular. Though we shouldn't forget, that when the Crimson Court came out, the patch for the main game did nerf some items to accommodate the new items introduced by the expansion.
EDIT: I searched around and the date I'm finding is June 19th, which is exactly one year after Crimson Court(June 19th 2017). We'll see if they manage to hit the deadline.