You don't need macOS Mojave to download Alto's Adventure, however, since the game is out on the Mac App Store now for $9.99 [Direct Link]. The game follows Alto as he journeys across various environments on an endless snowboarding adventure, where players have to guide Alto and chain together combos using a one-trick button system.
Alto's Adventure first launched for iOS in February 2015, and was followed up this year with a sequel called Alto's Odyssey. The sequel is still an endless runner, but introduces new features, controls, and a new desert location. Snowman says there's a good chance that Alto's Odyssey will appear on macOS as well.