It would be great to have an API to permitt the erase of history. Example of use case: KeepassXC n seconds after after password copying, it erases systemd clipboard, but Klipper instead still keeps the history, so you will have a leak of previously copied passwords.
Sorry, but what keepassx does is just stupid and doesn't work. The clipboard on X11 is broadcasted. If one wants to ensure that other applications cannot read it the approach just cannot work. Any application can listen to the clipboard changes. Given that I don't see a point in adding support for something which is broken anyway.