Dolphin has the the nice feature to add the hamburger icon to the tool bar when the menu bar his hidden and vice versa. This would enhance the UX in other KDE apps as well, which show a toolbar, such as Okular, Kate, etc. If no toolbar is present, a warning dialog should be displayed, like it's already the case for Kate: "This will hide the menu bar completely. You can show it again by typing Ctrl+M. [x] never remind again. [OK]".
Kirigami already has a feature sort of like this for its apps, but in order to implement this in all QWidgets-based apps, it would have to be a feature in KXMLGui. ...Which seems reasonable to me. Moving it there.
*** Bug 211304 has been marked as a duplicate of this bug. ***