Can't be that hard, games as old as just cause 2 have it. From my understanding all the windows store games so far dont have ultrawide, gears of war or quantum break.
I get thats its a niche market and its not even 1% of owners on steam but its the same as multi monitor. Which doesnt work on windows games either.
It depends on the engine. A lot of game engines don't really support ultrawide and if they just change some settings to allow you to use 21:9, it will mess up the FOV (it will be similar to this). Since Forza is pretty much an Xbone game, they didn't make the engine with ultrawide in mind since all modern TVs are 16:9.
Could they bring in 21:9? Of course they can but it will look really weird unless they spend a lot of time and money editing their engine to support it properly. I'm sure 21:9 will be supported eventually. It looks like the Forza series is going to be the main "look at what DX12 can do" game.