I’m starting to wonder if my installation is cursed or if the spacer widget for plasma truly is missing? Because at the time of writing I can not find it when adding widgets - painstakingly reading every widget or searching yields the same result, there is no spacer widget.
On Fedora 43 with KDE 6.6, I have to
- enter edit mode,
- click a panel (which opens the panel settings window, where you can change position, auto-hide, etc.), and
- then in this panel settings window, there is a button in the top right, with a plus and a text “Add new” (or something along the lines)
This button is actually a drop down and combines the previously separate “Add spacer” and “Add widgets” buttons in the top right.
So you found it? KDE is a maze sometimes (like to edit the panel or themes for day/night etc.). But at least basically everything can be done 
-edit: ops, I mistakenly took you for the OP.
My hero!
My only previous experience with KDE and plasma was on Kubuntu and apparently they add it as widget in the “normal” list of widgets as well.
And in the widgets part we can only add or remove all. To remove just one instance it’s in another different place (I think in the widgets themselves in edit mode or something like that).