Unfortunately, due to recent changes in Plasma, WallpaperSwitch no longer works. Please read on for an explanation and an alternative solution.
One of the changes to the Plasma workspace in Plasma 5 (based on KDE Frameworks 5) is that the option of having separate Plasma widgets on each virtual desktop is now gone. This is unfortunate, because one useful side effect of being able to do that was the ability to have different desktop backgrounds (wallpaper) on different virtual desktops, so proving a useful visual cue as to the desktop currently displayed.
The most relevant KDE bug is https://bugs.kde.org/show_bug.cgi?id=341143
The position of the Plasma developers, as stated in the evaluation of that bug, is that there have been major architectural changes within Plasma and at the moment the option is not available, with no simple way to bring it back. It has been acknowledged as an omission and is currently being worked on. Please be considerate to the developers and refrain from commenting on this bug unless you really do have a useful contribution to make to the discussion.
WallpaperSwitch attempted to bring back the ability to have different wallpapers on different virtual desktops, pending the feature being reintroduced into Plasma. It allowed a wallpaper image file to be configured for each virtual desktop, changing the background when the virtual desktop changes. It did that by changing the wallpaper slideshow image available.
Unfortunately, there have been changes within the Plasma desktop slideshow plugin, in order to allow selection of multiple images and displaying them in a configurable order. This was implemented by commit ea32a761 to plasma-workspace on the 25th July 2019. The change is useful, but as a side effect it no longer notices changes to the slideshow images so this wallpaper switching method will no longer work. If you are using KDE distro packages then this change may not have made it to your updates yet, but it will surely do so soon.
If you still wish to have different wallpaper images per virtual desktop, then I suggest that you install the "Vallpaper" plugin and set it as the desktop background. The configuration dialogue will then allow you to select an image file (or more than one image) for each virtual desktop.
You can obtain this plugin via GitHub, or by using the "Get New Plugins" button in the desktop configuration dialogue. I have no connection with Vallpaper, apart from being able to report that it appears to work well and does exactly what it says.
Jonathan Marten, http://github.com/martenjj