Using KDE Beta on Tumbleweed

If you want to use beta releases of KDE software on Tumbleweed then you need to add the extra repos, take note that they need to have higher priority (lower number) than default repos. 

sudo zypper ar -fp 75 https://download.opensuse.org/repositories/KDE:/Frameworks/openSUSE_Tumbleweed/ KDE_Beta

And 

sudo zypper ar -fp 75 https://download.opensuse.org/repositories/KDE:/Applications/KDE_Frameworks5_openSUSE_Tumbleweed/ KDE_Beta_Apps

then perform 

sudo zypper -v dup --allow-vendor-change

And reboot

Source

KDE is the project that is responsible for (among other things) Plasma desktop