De acordo com anúncios , você precisa adicionar o Widget de Plasma correspondente ou o botão de menu de " Decorações de janela "configurações.
Global Menus have returned. KDE's pioneering feature to separate the menu bar from the application window allows for a new user interface paradigm, with either a Plasma Widget showing the menu, or with the menu neatly tucked away in the window title bar. Setting it up has been greatly simplified in Plasma 5.12: as soon as you add the Global Menu widget or title bar button, the required background service gets started automatically. No need to reload the desktop or click any confirmation buttons!
Funciona na minha máquina