Removing items installed from the web using "Get New....." using the remove button in the theme window for said item, leaves an entry in "Get New Hot Stuff" window, under the installed section, of the already deleted item. Uninstalling items from "Get New Hot Stuff" window directly does deletes the items and entry properly. This apply to icon, mouse, sddm, window decoration themes.
I can confirm. The following things are affected on my neon dev unstable: icon theme cursor theme sddm theme desktop theme k3b theme
For recent versions of kde this becomes a non issue. Most of the get new stuff, stuff, got reworked and now there is an undo function that works is a similar manner as i described on this bug.