This is incorrect. Just for every-ones information virtual desktops are a part of Plasma5 desktop. FWIW I’ve been told specifically by TPG that OpenMandriva doesn’t really do much customization of KDE packages. Virtual desktop settings (Workspace>Desktop Behavior) are still in SystemSettings5 because KDE developers put them there. Activities are not the same thing as Virtual Desktops. There are plans to fix this here. You can see from the length of that bug report and this one that a lot of users share your opinion, myself included. In the bug reports it is explained why KDE devs saw fit to do this. KDE bug 343246 also explains relationship between activities and virtual desktops.
I have openSUSE Leap/Plasma5 and have tested Plasma5 on other distros and have never found virtual desktops missing.