Additionally, see my other comment about accessibility, which is also negatively impacted by hardcoded UI appearances.
Since customization where pretty standard back then, it feels like product managers pushing their "vision" rather than technical difficulties preventing that nowadays.
I think it was on the real desktop, not some bitmap, but not too sure.
I wish Gnome folks did not keep on copying it more and more. I see why it's easier to do, but KDE somehow manages.
(And if one thinks that Apple UX design can't go wrong, remember Macbooks from a couple of years ago.)
macOS still has several traditional desktop affordances that are eschewed by GNOME, like full menus (not just hamburger junk drawer menus) and customizable toolbars.