Based on the version number (v1.35.1.3s) and the mention of 21 DLCs, this report refers to a specific snapshot in the history of American Truck Simulator (ATS). This version was released around mid-2019.
Here is an interesting report analyzing the state of the game at that specific version, why it was a pivotal moment, and what that collection of DLCs represented. American Truck Simulator -v1.35.1.3s 21 DLCs...
Believe it or not, some players prefer the economic balance before Utah was added. With only Arizona, New Mexico, Oregon, and Washington, the in-game economy had a tighter loop. Short-haul jobs between Portland and Seattle were profitable, and the lack of vast, empty desert routes (Utah/Colorado) meant more scenic variety per square mile. The 21 DLCs provide maximum variety without the sprawl. Based on the version number ( v1
Playing this specific version offers a unique "time capsule" experience compared to the modern game: Arizona (Free – but still counts as DLC)