A serious question.
I went totally blind long before Win8.x & have never seen the modern UI, nor whatever Win10 has done. Keeping that in mind, I'm seriously wondering what is meant by a "flat" UI.
Are you talking a nested tree of +'s & -'s to expand or collapse a set of navigation sub entries? Just plain text on plain buttons with no shading, iconography, nor embellishments? No 3D, background images, wallpaper, animated GIF's, or stuff like that?
When I hear of a flat UI it makes me think of a simple list of links to same page content, other page content on the same site, or links to off site content. Put them in a Navigation Menu block at the top of the page, a Back to top link at the bottom, & you've just made an intuitive UI. Don't hide any of it, don't make it visually pretty by hiding links/content behind blobs of scripting, just give the potential customer what they're looking for so they can find it quickly, easily, & most importantly, in an Accessible fashion that doesn't cause a screen reader to shit itself.
I know you sighted folks like the pretty stuff, I used to myself, but having turned blind I'm now all too aware of how badly some of that visual only pretty stuff turns into headaches for anyone not equipped with perfect vision.
So what's meant by a "flat" UI? Anyone care to try & describe it?
*Leaves a pint as a thank you gift for anyone whom does so*