Yep. They were added a while ago.
Folks from the forums have created game modes that could easily be implemented as actual game modes, probably enough to create a public community Playlist for players to play eachothers modes. (Similar to halo)
They are a fun way to change the game, just very difficult to get 5 willing people to get together in a single private match lobby to play with said modifiers.
When the game first came out, you had too many people wanting to play, and had to break the lobbies up, now it seems like nobody wants to play : /

