Forza Horizon 3 is reaching what Microsoft calls 'End of Life' and will no longer be available to buy either on Xbox or PC after the 27th of September, but the good news is if you do want to pick up the game before then, it's incredibly cheap to do so.
Even though the game will no longer be sold after this date, if you already own it you'll still be able to download and play it, and the servers and online functionality will remain for some time to come yet. This is pretty typical of the Forza Horizon series, with the first two games no longer supported either.
If you do want to buy the game then it's on sale for just $9.90 for the standard edition, and $21 for the Ultimate Edition, with all the DLC also on sale.
If you love Forza Horizon 4, then it's worth picking this up too if you've never done so - it's an amazing romp across Australia with a huge selection of cars, races, and challenges to compete in.