According to Forza Motorsport the game will reach "end of life" status on 30 September 2018.

What it means, they are removing the game and ALL DLC from their xbox services, so you will not be able to buy the game anymore.

Currently it is part of the [August Games with Gold], so grab it whilst you can.If you do own the game already, you will still be able to download and play the game and all DLC.

Unfortunately Microsoft hasn't said why they are delisting the game, but it seems to be a trend, since they did the same with Forza Horizon 1 in 2016.