I think windows 95/98 era was the peak of UI discoverability. As a kid, I and my friends could learn and explore how a program worked just by expanding all the menus and opening the various dialogs. Dialogs which were filled with fields and their descriptions! Before then, it was the world of DOS and you had to know what you were doing ahead of time. And now, UIs are dynamic, initially hiding stuff from the user and…
You can put a lot more on a desktop than on a mobile screen. In the Windows 95/98 era, the standard screen size was no less than 14 inches, now it is around 6 inches, and the mouse is a much more precise instrument than your fingers. On mobile, if your app becomes complex and you don't want tiny, hard to see, hard to reach targets, you have to hide things.
Now, modern desktop apps often mirror the mobile app, even though they have a lot more space available. It makes sense, not only developing two entirely different UI is much more expensive, but it also makes harder to transfer your experience from the mobile to desktop and vice versa.
There is no easy solution, and it looks like things are starting to settle a bit, maybe a few years from now, we will see a real common design language. The problem with right slide is not the hidden screen, it is that every app seems to have a different interpretation of what to do for that gesture, if they do anything at all. If every app does the same, it will be as "obvious" as right clicking to open a context menu today.