Apparently, importing ~/.config/kglobalshortcutsrc is enough for the shortcut to show up in the System Settings, but not enough to load the shortcut. Disabling, applying and re-enabling and applying again also doesn’t help, and neither does re-assigning to the same shortcut, which resulted in this lovely error dialog.
Its definitely not common. But especially in Unix-like software, referring to the Windows key as Meta introduces unnecessary confusion, especially to newcomers.
Prime example: a lot of newcomers use Nano as their terminal text editor. It refers to the Alt key with the abbreviation M for Meta. So someone who’s already taking a huge plunge into a vast and complicated technical topic has to first learn that M means Meta which means Alt and then also has to learn that in other contexts, Meta means the Windows key.
Now, maybe the solution is for Nano to change, not Plasma. But one way or another, it should be standardized.