I have four virtual desktops and want to use slideshows as background.

But it seems to be impossible to choose different slideshows for different virtual desktops.
All desktops share the same slideshow.

In older versions of kde it was very easy and intuitive to configure the slideshows for each
virtual desktop seperately.

Maybe I overlooked something. It seems to me rather unlikely that the developers decided
to cancel the possibility of different slideshows.

Different backgrounds for virtual desktops has been removed quite some time ago, you should use Activities for this.