How to Disable the Unicode Insert Shortcut
General
The Ctrl+Shift+U shortcut is interfering with the toggle case key of many editors.
The shortcut key is not part of the KDE desktop environment but is part of the ibus system.
Disabling/Removing the Shortcut
Install the application for configuring shortcuts for inputs from the ibus system.
Run the ibus-setup command as a regular user to start the GUI application.
Open the Emoji tab end delete or change the Unicode code point: entry.
Restart the ibus daemon to make it have effect.
If this does not have effect logout and login from the desktop.