A post on Reddit discusses the disappearance of the store listing for Dark and Darker from Steam. There is a subsequent edit sharing an explanation from the developer IRONMACE's Discord saying the game was hit by a cease and desist letter and DMCA takedown by Nexon. The background for this can be found on Eurogamer, which previously reported on a police raid following allegations of stolen source code and other malfeasance. Here's the explanation:

As Victoria explained for us earlier this month, back in August 2021, a former Nexon employee was allegedly reported for leaking company data and assets while they were working from home during the Covid-19 pandemic. This employee - known as Leader A - was heading up development of a project dubbed "P3". However, after allegedly being caught leaking "thousands" of the project's file, Leader A was fired from Nexon.

Since then, half of the 20-person P3 team quit Nexon and joined Ironmace to work on Dark and Darker. This included the developer allegedly caught - and subsequently fired for - leaking the P3 files. There have since been claims that P3 was used as a prototype for Dark and Darker.

According to Nexon, Dark and Darker has "striking similarities with the concept for P3 in major gameplay, rules and arts, which could not have been made independently" and the company confirmed that it would "take firm measures not only against Leader A but anyone else or any other company related to the data leak".