Slightly Mad Studios announced a month ago that Project Cars 2, one of the best racing sims ever, was being removed from Steam (opens in new tab) on September 21. I hope you were able to grab it if you wanted it, because today is the day and sure enough, it's gone.
There's been no further word from Slightly Mad Studios about the removal, but a notice on the Project Cars 2 Steam page (opens in new tab) states that it is no longer available for purchase. SteamDB (opens in new tab) carries a similar message: "At the request of the publisher, this app has been retired. You may not be able to buy or get it."
It's fair to say that "may not" in this case means "cannot." You might be able to scrounge one up from a key reseller if you're especially eager and willing to roll the dice, but the Humble Store (which offered the game via Steam keys) has also removed its Project Cars 2 listing, and the Project Cars 2 website (opens in new tab) no longer offers purchase options either.
The reason for the removal of Project Cars 2 (and soon, the original as well) is simple, and one we've seen before: Expiring licenses for cars and tracks. Simply put, developers and publishers pay for the right to use real-world products in their games—in this case, cars and tracks—in order to enhance "authenticity." But those rights don't exist in perpetuity, and when the licensing deal expires, a choice must be made: Renew or let it go. And if sales aren't likely to justify the expense—which is all but certainly going to be the case for niche, years-old driving sims—then the deal will be left to lapse.