Duet Night Abyss was supposed to be released on PC on Steam at the end of December. However, something went wrong and the release was postponed.
The page was not removed from Steam, and the developers published a statement stating that they must comply with the "platform requirements" and that additional time will be required:
[...] we are forced to announce that the release on Steam has been postponed, as additional time is required to complete the necessary procedures. This decision was made in order to comply with platform requirements and ensure a stable gaming experience. We understand that this news may disappoint players who were looking forward to the release of the game on Steam, and we sincerely apologize for any inconvenience caused. Please be assured that the game content is fully ready, and preparations for the release on Steam are actively continuing.
At the same time, Duet Night Abyss is already available in Steam's competitor — Epic Games Store.