I use Kjots everyday, all day long. I have important information there and the lack of an undo feature has often ruined my day. Whenever I accidentally delete some lines I immediately go to Edit - Undo and, well, you know. It's not there.
I know that I a fool for accidentally deleting lines, but I think every app should be designed keeping in mind that the world is crowded with fools like myself.

Feature requests can be made at bugs.kde.org

Curiously, the Undo feature has been implemented in Kjots,
BUT
there is no Undo option in the menus,
nor is there and Undo button available for the toolbar,
which means that I found out about its existence only accidentally!