[1] http://interface.free.fr/Archives/Apple_HIGuidelines.pdf [2] https://developer.apple.com/library/content/documentation/Us...
Problem: You plug in an external monitor and for some reason it doesn't immediately display an images. What do you do?
In pre-el-Capitan, you open up the display settings pane and press the button marked "Detect Displays".
In El Capitan, you open up the display settings pane and note that the button is not there. Are you screwed? It appears so. So you run a google search and discover that if you hold down the option key, the button that had been there for maybe 16 years magically appears in an area of what previously appeared to be blank in the dialog.
I believe that's called the Indiana Jones UI pattern.[0]
So yes, the discoverability has gotten worse over the years. This is something that was visible and made invisible.
The trash icon was there in the Dock in 10.5, and still there in 10.11. The search function in help was there in 10.5, and still there in 10.11. To remove things from the Dock, right click the icon and the "Remove from dock" is still there in both versions. Any menu items that will open a dialog still indicates with three dots ("...") in both versions. Shortcut for opening dictionary is still not very discoverable in both versions (Cmd-Ctrl-D). I still don't know how to trigger Space/Expose without either using gesture (in 10.11) or look up the shortcut key (in 10.5).
I always see people saying Apple UI got worse in discoverability for the past few years and make it sounds like earlier releases were perfect, but I don't believe that is true. The bad part of the UI interaction was always there (e.g. drag icon from the dock to somewhere to remove it). The main difference I see is that 10.11 has a LOT more features than the 10.5, but the core interaction remain the same in regard to discoverability. In 10.11, we're exposed more to gestures, because it's quick, but there is always alternative way to trigger something without gesture.