I once spent a lot of my time and the time and the time of a developer trying to find a setting because there was no indication that a window had more content (a checkbox) to scroll down to. Something that would have been obvious before the onslaught of hidden scrollbars. The trouble is that having my pointer over the navigation pane--practically a guaranteed position--causes the scrollbar on the other pane to be hidden. Without the visual cue of a scrollbar there was no reason to move my pointer over to the other pane to discover there's more. Hell you might not even know it's a separate pane now that we've gotten rid of every defining border. I shared a screenshot with the developer, assured them that I was using the current version, only to have him say "scroll down." No doubt, I'm the fucking idiot (/s).
Just like on mobile, you're supposed randomly interact with every UI element in hopes of discovering how it works only to have that learned skill be unique to one fucking app. Tap it, slow tap it, slow tap it for a different amount of time, tap it faster, spam it... "google it"... oh, this time you're supposed to drag it to something that doesn't even look like a UI element. Stupid grandpas!
"Is the checkbox checked?" was never as ambiguous as "is the slider switch on?" Also, the checkbox uses less screen space! I'd argue that they optimized for neither screen space or user friendliness. It's optimized for a look and you can even make it worse by making it flatter. Go ahead make it look like two squares! Is the darker area the switch part? Who cares! It looks so clean and distraction free! I was so distracted by knowing what state the switch was in.
Sorry, time for my meds. I usually make it half way through the day.