@Ronaldo There's no official info regarding that yet. I have an ultrawide monitor, so I really want to see this too. Make sure to send Feedback from the Stadia app to make sure Google is aware of it. The more feedback they get regarding this subject, the better chance it's being implemented.
The missing feature is one of the reasons I am likely to cancel my Stadia and never use it. I want to be involved in BG:III early access. If I can't use my ultra-wide I guess I'm buying it elsewhere and moving on.
I also want to see ultra wide, across three 1080p monitors. This option would still be less bandwidth than 4k.
But maybe eventually even three 4k monitors.
This is how I want to play Breakpoint