What Dota 2 feature added in 2015 lets players use community-created game modes? The answer is Arcade.
Arcade is Dota 2's hub for custom games and community-created modes. It grew out of the 2015 Dota 2 Reborn update, which moved the game to Valve's Source 2 engine and expanded support for player-made experiences beyond standard Dota matches.
These modes can alter the rules, objectives, maps, heroes, or win conditions. They are descendants of the custom-map culture that made the original Defense of the Ancients famous inside Warcraft III. That community-driven tradition was important to Dota's identity, so custom games became a major part of Dota 2's ecosystem.
The common confusion is with Watch, Armory, or Compendium. Watch is for viewing matches, Armory concerns in-game items, and Compendium-style features have related to events and tournament content. Arcade specifically refers to the area for browsing and playing custom games.