Forza Horizon and all of its DLC will be removed from sale on Xbox 360 on October 20, 2016. The game will still be available for those who wish to purchase a disc version, but the digital version and any and all DLC you may desire to obtain will have been removed by that time.
Those who already own the game and any DLC will still have access to download the content, just not obtain new content for it. This delisting also happened to DLC for Forza Motorsport 4 in September of 2015.
While we’re reveling in the euphoria of launching a brand-new Forza game, I do have an important update regarding the original Forza Horizon. On October 20, the original Forza Horizon for Xbox 360 will reach “End of Life” status. That means that the game and its associated DLC will no longer be available for purchase. Players who own Forza Horizon before that date will still be able to download and play the game and its associated content as normal. After that date, however, Forza Horizon and its associated DLC will no longer be for sale.