It seems that the KDE folk upstream have made available a replacement
for the desktop cube effect.
I'm told it's at https://github.com/zzag/kwin-effects-cube.git, but
building it for openSUSE is beyond my skills. Is there any chance that
it might be packaged and distributed for Leap and Tumbleweed as an
Wellington "Harbour City"
Unless further notice, openSUSE Leap 15.3 support will end in a few days. 
If you're using packages from the KDE repositories (KDE:Frameworks5,
KDE:Extra, etc...), you have a few days left to upgrade to Leap 15.4.