I believe it's just a temporary issue.
But if you feel like it, open a support ticket as this game should be fully supported until september this year. (they answer all the tickets now)
https://support.forzamotorsport.net/After that they will answer you saying they can only support online gameplay and leaderboards, like they are doing with the older games including Horizon 2 and older.
Also, Forza 6 will probably be Gold this year, the month before delisting, so the game will likely be fully working around that time.