Yes. Every Steam key for ETS2 unlocks all versions. Use the Beta tab in Steam properties to downgrade to 1.15.1 specifically.
If you need version 1.15.1 for mod compatibility or nostalgia, here is the safe and legal method: Euro Truck Simulator 2 1-15 1 Activation Key
At first glance, this looks like a standard software license request. But for veterans of SCS Software’s masterpiece, this string of numbers represents a specific era of the game—version 1.15.1—a build that brought revolutionary changes to cargo markets and trailer physics. the prompt appeared
He cracked the seal and slid the disc into the tray. The drive whirred like a jet engine. Then, the prompt appeared, cold and demanding: Enter Product Activation Key. Euro Truck Simulator 2 1-15 1 Activation Key
Introduced a new exclusive interior for this truck model.