Here is the full list of all 41 Steep achievements.
The base game contains 29 achievements, and there are 2 DLC packs containing 12 achievements.
Defeat "The Beast" during Winterfest
Defeat "The Dragon" during Winterfest
Defeat "The Fratelli Twins" during Winterfest
Defeat "Super Savoyard" during Winterfest
Defeat "Lincoln Black" during Winterfest
Stay on the Sled without falling for 30 seconds (Winterfest DLC required)
Obtain the Gold medal in 10 Challenges of Extreme Pack
Obtain the Gold medal in 5 Challenges of Extreme Pack
Finish the Trial Rogue 4 of Extreme Pack
Take off and land 5 times in the same Speed Riding run (Extreme Pack DLC required)
Fly for 5 minutes with the Rocket Wingsuit (Extreme Pack DLC required)
Safely land after jumping from all the 7 Base Jump Spots in Alaska (Extreme Pack DLC required)