Update: Shortly after sales of The Sinking City resumed on Steam, they were halted again: The listing is still present, but it can no longer be purchased. There's also still no sign of it on GOG, but it is now relisted and available for purchase on the Microsoft Store.
In an email, Nacon explained that the "online stakes" referenced in its initial statement was not about in-game content, but distribution platforms. The publisher said that it is Frogware's responsibility to return the game to at least some of the storefronts in question. and that it has no control over the timing of the return, which is presumably why you'll see it appear on some distribution platforms ahead of others.
As for why The Sinking City re-appeared on Steam and then was taken down again, that's not clear. I've emailed Frogwares for more information, and will update if I receive a reply.
The Sinking City is Sherlock Holmes by way of HP Lovecraft: A 1920s detective game set in a flooded, dilapidated fishing town built atop caverns filled with otherworldly horrors and madness. It wasn't great—"an occasionally entertaining detective game blighted by poor writing, rote combat, and a dreary open world" is how we summarized it in our 2019 review—but it resurfaced a year later in an unexpected way when it very suddenly disappeared from most digital storefronts.
In August 2020, developer Frogwares released a statement saying that the removal was the result of an ugly dispute with publisher Nacon, formerly Bigben Interactive. Frogwares alleged that Nacon withheld promised financing during development and played fast-and-loose with post-release sales figures; Nacon, in a reply, "emphatically" rejected the allegations and said Frogwares was "seeking to discredit Nacon in the eyes of the public and professionals alike" with its public statement.
The whole thing went before French courts to sort out, and while that process is still being resolved, Nacon issued a statement today claiming an interim victory: "The Court ruled that Frogwares had terminated the contract in a 'manifestly unlawful' manner and, as a result, ordered, as a 'precautionary measure, the continuation of the contract (...) until its term or until a decision is made on the breach of this contract and ordered Frogwares Ireland to refrain from any action on the breach of this contract and ordered Frogwares Ireland to refrain from any action that impedes this continuation."
As a result of that ruling, The Sinking City and its various DLC bits have now been returned to Steam. Interestingly, and very vaguely, Nacon suggested that some elements of the game may be absent or incomplete, saying, "Insofar as some of the game's online stakes depend on the goodwill of Frogwares to perform, their absence cannot be attributed to Nacon."
(Another interesting point that several fans have noticed is that while The Sinking City is currently on sale for half price on Origin, where it's remained available for purchase all along, the Steam re-release is carrying the full $50 price tag.)
The Sinking City is a singleplayer investigative adventure game, so it's not clear what sort of "online stakes" Nacon may be referring to—it could be some aspect of DLC functionality (bonus quests, cosmetics, and other in-game items are all available for separate purchase), or it might also be nothing more than generic "not our fault" boilerplate. I've emailed Nacon for more information, and have reached out to Frogwares for comment as well.