SUMMARY
In previous versions of Plasma, when the Display Configuration UI had a drop-down to select the primary display, one of the options was "No Primary Display". With the new Display Configuration (at least from 5.18, I believe) the primary display selection is a checkbox on each device, which - when checked in a device configuration - automatically unchecks the "primary" checkbox on all other devices, but the "primary" checkbox cannot be unchecked manually - clicking it again doesn't do anything.

But double clicking the "primary" checkbox can uncheck it, though that does not enable the "Apply" button, so that configuration cannot be applied.

STEPS TO REPRODUCE
1. Have multiple monitors connected.
2. Make sure one of the devices has the "primary" checkbox checked.
3. Double click the "primary" checkbox.

OBSERVED RESULT
The checkbox gets cleared, but this configuration cannot be applied.

EXPECTED RESULT
Probably the checkbox should not be cleared.

SOFTWARE/OS VERSIONS
Linux/KDE Plasma: 
KDE Plasma Version: 5.20.80
KDE Frameworks Version: 5.77.0
Qt Version: 5.15.1

ADDITIONAL INFORMATION
I expect that this is a minor UI issue and the correct behavior is that there always must be a primary screen (unlike as it was in older Plasma versions).