Whoops -- it looks like someone pulled the trigger early on the Steam store page for Sekiro: Shadows Die Twice. It's up right now, and most notably, the release date for the game is there for all to see. According to this page, FromSoftware's action/adventure title will launch on 22nd March 2019.
Sekiro is confirmed to be present at Gamescom 2018, which gets started tomorrow. It's entirely possible that the release date is part of Activision's plans for the game during the show, so we imagine this date will be confirmed very soon indeed. For now, we'll call it a rumour, just to be on the safe side. As always, we'll keep you up to date with any new information.
Update: FromSoftware has confirmed the release date for Sekiro -- 22nd March 2019.