When a game reaches "End of Life", that means it and its DLC will no longer be available for purchase in the Microsoft Store. However, features like multiplayer and online services will still be accessible for those that own the game.
Forza Horizon 3 will reach 'end of life' status on September 27, 2020. That means that the game and its associated DLC will no longer be available for purchase after that date.
After September 27, 2020, players who own Forza Horizon 3 will still be able to download and play the game and its associated content as normal.