Windows NT (way back to its original 3.1 release) always had the idea of multiple independent desktops where applications can't cross-interact. If you use runas to run GUI applications under different user contexts, you might notice that some things like drag-and-drop and the clipboard don't work, and also screen recorders won't work ... Most users never use this functionality normally, but you might notice sometimes if you do "Run As Administrator" and the applications don't quite integrate with non-admin apps.

Oh, so that's how this works. I've noticed this through occasionally broken copy-paste between normal and "elevated" apps. Never realized it's a separate feature, not just a forced sandboxing between regular and "elevated" apps. Thanks!

In store applications without full trust permissions you can not.

Store applications are not the only applications on Windows 10. Closing holes in store applications alone does not close all the holes.

This article is addressing sandboxed applications.

Regular applications, yes, you can pretty much do whatever in userspace in Win/Mac/X11. If you're going to create a new sandbox environment for applications (Windows Store/MacOS store) in today's world, it really needs to be locked down with explicit permissions for each type of I/O access.