I can confirm that in Kubuntu 20.04 LTS, Plasma 5.18.5 Ctrl-D will delete a file on the desktop (accessed in the desktop view, not in the file manager). The file has to be selected and the desktop view (not any window) have the focus. (Considering the interaction with the terminal as described in question, that happens only if the terminal is not maximized nor full-screen and it loses focus in favor of the desktop by clicking a file there.)
I don't have an answer on how to remove it, and I think this must be a bug, although non-reported and completely unnoticed until here. I have initially thought of it as a remnant of an old feature that was forgotten about by the KDE people, but in the end I think it's simply a sudden bug around version 5.18 of Plasma - for the following reasons:
Absent in Kubuntu 20.04/Plasma 5.18 settings, that shortcut is not documented. It doesn't appear in the context menu when selecting the file either.
I couldn't find anything about it on the internet at all. (It might have been absent before Plasma 5.18, although it's hard to test now).
It was removed in more recent versions of Plasma. (Tested with Plasma 5.23 in Kubuntu 22.04 daily, Fedora and Neon. Cannot be sure about Plasma above 5.18 and before 5.23).
It is also clear it never made much sense. Beside its confusion with the terminal key, it provides nothing more than the Delete key, which (more economically) does the same, and more: unlike Delete, Ctrl-D only works on the desktop, and not in the file manager, etc.
