the game is not on Steam anymore
The game is still on Steam. It's just 'not for sale' anymore. The developer even stated that they pulled the sale of the game, not Steam. So it's entirely possible the developer is still available to put the game back for sale or offer ways to get steam/product keys to redeem the game on steam without buying it through steam, etc.
https://store.steampowered.com/app/3299570/No_Mercy/
https://store.steampowered.com/news/app/3299570/view/588390482275467287?l=english
It's similar to how other video game companies branch away from steam, in which the game publisher decides to go with another platform to sell on instead, so they just stop selling on Steam, but still keep it on steam for people who have bought it there.