Edited September 13, 2021
at 05:53 AM
Link to deal [microsoft.com]
Link to info on it being delisted [forzamotorsport.net]
Sale ends in 48 days, this will be the last sale for the game before it goes bye bye. It's a Play Anywhere title, so purchasing this will get you the Xbox and PC versions.
Heads-up, if you didn't pre-order Forza Horizon 4 you also won't have the Formula Drift Car Pack, it's not included in the Ultimate Edition, so if you're a completionist you'll have to pick that up too. Link here [microsoft.com]
There are also a bunch of free cars to grab, here's a partial list:
Free car DLC and other freebies [microsoft.com]
in case the list above missed any.
Edit: this sale is over and the game and all DLC are being delisted in 2 days, now's your last chance to pick it up.