On a fresh install of KDE Plasma, the "Clear History" button of the clipboard widget becomes permanently nonfunctional if one selects "No" and "Do not ask again" in the following confirmation dialog.
STEPS TO REPRODUCE
1. Copy something to clipboard
2. Open the Clipboard widget, Click the "Clear History" button
3. In the confirmation dialog, check "Do not ask again" and click "No"
"Clear History" button becomes nonfunctional. History cannot be cleared anymore.
Functionality can be restored by manually editing ~/.config/plasmashellrc and removing the line "really_clear_histoy=false"
When clicking "Clear History", the confirmation dialog appears again.
Linux/KDE Plasma: 5.11
(available in About System)
KDE Plasma Version: 5.21.3
KDE Frameworks Version: 5.80.0
Qt Version: 5.15.2
*** This bug has been marked as a duplicate of bug 336053 ***