Nobody really has. Apple comes the closest but they keep rug pulling it in weird ways.
Windows and Mac in the 90s had very consistent GUIs with such consistency in things like keyboard shortcuts that apps could easily be learned. The term “intuitive” was king in the realm of UI design.
I thought we're not discussing whether we like one UI over another but whether an OS's UI was (internally) consistent. You may dislike KDE, but it uses it's design language all throughout the UI from window manager, the application launcher, the settings dialog, everywhere. Windows famously has three (or more?) styles of control panel UI. You may prefer that, and to each their own, but it is inconsistent, which was the topic of this thread.
KDE has the newer QML-based Kirigami and older Qt Widgets frameworks that are not consistent. Widgets apps absolutely look like actual desktop apps (menus, toolbars, dialogs), Kirigami apps look like mobile apps (nav bars, hamburger menus, page-based navigation). There is definitely a visual and functional inconsistency between the two, even if they use the same theme pretty well.
Problem is, both Firefox and Chrome look out of place on it when you use a non-standard color scheme. Custom keyboard shortcuts won't work on Chrome. When you change a theme from light to dark and vice versa, both browsers like to have text in their UI stuck on the wrong color.
I could use Konqueror, I guess, but its ad blocking plugin (and plugins overall) seems to never have progressed much since KDE 2.
Right, applications can break all sorts of UI consistency. But the question is: do the standard components that the vendor ships do it? I argue: KDE is consistent. I don't want my desktop UI to dictate how an app draws its UI (or games would be impossible).
Even games need it, missing input field features has plagued PC games for decades and can be crippling for input method users, Skyrim's console needs mods to support copy and paste. Custom mouse acceleration curves is the reason everyone disabled it, zero acceleration is the easiest way to make different games handle mouse input consistently.
Windows and Mac in the 90s had very consistent GUIs with such consistency in things like keyboard shortcuts that apps could easily be learned. The term “intuitive” was king in the realm of UI design.
Then the web hit and all that died.