This is a wild, ongoing story that is playing out in a rather bizarrely.
Basically, Frogwares signed an agreement with Nacon (formerly Big Ben Interactive) to license and publish their game in return for funding, but Frogwares still owned the IP. Frogwares claims that Big Ben was consistently late with payments and did not honour milestones for further funding. They took on an EGS exclusivity agreement to help get funding. At some point during the development, Nacom bought another studio and then demanded that Frogwares give their source code to this new company, which Frogwares refused to do.
The released the game in June 2019 but then were told by Nacom that the milestones that were previously agreed to were cancelled, therefore Frogwares would see no profit from the game. Frogware filed a lawsuit and finally got access to some sales data but found it wanting in many regards. They also found that copyright notices on the game were incorrect with the result of misallocating the IP ownership. Their logo was removed from the PS4 and X1 versions of the game and they discovered that Nacon was presenting themselves as the IP owners for the game, and had bought domains for Frogwares' other Sherlock Holmes titles (most of which was published by Focus Home Interactive, with some published by Atlus in North America or self-published by Frogwares).
Frogwares believes they had what they needed to terminate the contract with Nacon, despite some complications with the French legal system due to the COVID pandemic, and so they pulled their games from various storefronts alongside this letter.
In January 2021, that French legal complication played out against them when the Paris Court of Appeals determined that Frogwares acted unlawfully and decided that Nacon would be able to put the game back on the market.
Today, The Sinking City was put back on Steam with Frogwares listed as the developer, but Frogwares says they did not make this version. It seems to be an older version, missing much the DLC and features like achievements and cloud saves.