We added our first version of a Map Preference system in Update 17718. This gives you influence over the maps that you will encounter in ladder play.
Can I downvote maps in Quick Play or Ranked?
Yes to both! We have a separate preference window for each mode.
Are my downvotes shared among Quick Play and Ranked?
Nope - You have separate downvotes for each queue, since each queue features only the maps present in that queue.
Do I need to choose my downvotes before every match?
Nope, your map preferences are saved across both Quick Play and Ranked and will be persistent until you either change or reset your selections.
I downvoted Black Forest for team games but I still ended up playing on the map! What gives?
The map selection system chooses a map that has the least number of downvotes across everyone in the game. If every available map is downvoted at least once collectively, then you may end up on a map you downvoted individually.