Maybe it'll end up being true, but so far there is zero evidence. The only thing so far they've done that would in any way limit Steam is that their universal applications (what used to be called Metro) are Windows Store only. So you can't sell those on Steam. Ok, except nobody but MS makes those because nobody gives a shit. The "universal" part doesn't matter, MS's phones and tablets are in their final dying moments so there's no need to make something that runs both on real Windows and Windows RT/Phone.
At this point Win32/64 programs run better and have less limitations, and also have the advantage of running on all versions of Windows not just 10, so that is what people keep making. MS themselves are releasing their games using their new UWP format, of course, but nobody else seems to give a shit.
So it is a meaningless limitation for now. Programs using an API nobody uses won't work with Steam. Who cares? Other than that, nothing has changed or been limited. Steam runs great on Windows 10.
Will something change in the future? We'll have to wait and see. There's no evidence now though, because it hasn't happened. This is a doomsday prediction, and like most doomsday predictions it is based on what the predictor feels to be true, not actual evidence.