Come September 15, Forza Motorsport 6 will no longer be available for purchase on the Xbox Live Marketplace.
According to the game's official website, the game will enter "End of Life" status on that date, meaning both the game and all DLC will no longer be available for purchase. If you already own the game, you will still be able to play it.
Normally it would cost you $100 to get the entire DLC catalog. Instead, you can grab the Complete Add-ons Collection for $5 for a short time.
Xbox Live Gold members can play the game for free on the service through Games with Gold until September 15.
Forza 6 was released on September 15, 2015. The latest game in the series - Forza 7, has been available since October 2017.