After accidentally removing Plasma I've managed to install kde-plasma-desktop. Now the 'Breeze Dark' task switcher doesn't appear when set. All other switchers are working.

Operating System: Ubuntu 20.04
KDE Plasma Version: 5.18.8
KDE Frameworks Version: 5.68.0
Qt Version: 5.12.8
Kernel Version: 5.4.0-144-generic
OS Type: 64-bit