SUMMARY If you clean the klipper using the provided button, a popup appears wether you want to really clear it. There is also a checkbox which says something like "disable-this-popup" If you select this checkbox and click yes, it isn't possible anymore to delete the clipboard, just nothing will happen STEPS TO REPRODUCE 1. press delete button 2. enable checkbox to remember choice 3. click yes. 4. press delete button again, nothing is cleared OBSERVED RESULT EXPECTED RESULT the clipboard should be cleared and there should be a way to reset a "disable-this-popup"-dialog SOFTWARE/OS VERSIONS Windows: macOS: Linux/KDE Plasma: (available in About System) KDE Plasma Version: KDE Frameworks Version: Qt Version: ADDITIONAL INFORMATION
The issue is that there is a "dontAskAgainName" at https://github.com/KDE/plasma-workspace/blob/423308f854c4b76c72da4e3263614c7b20642ebf/klipper/klipper.cpp#L1001 If you enable the "dontAskAgain" feature and click no, so that the clipboard isn't cleared; there isn't any way to clean the clipboard. I think it should be impossible to enable dontAskAgain and clicking no at the same time, because it is very irritant.
To remove the saved setting, edit ~/.config/plasmashellrc and remove the line containing "really_clear_history=true"
*** This bug has been marked as a duplicate of bug 336053 ***