There is a similar question on Askubuntu. One solution was Maximus, however it doesn't work on KDE, only on Gnome family DEs (Gnome Shell, Unity, Cinnamon). KDE settings are rich, but i didn't find how to setup Maximus-like behavior, which unwraps windows to take all possible and space and also hides window header (window title). There is System Settings → Window Behavior → Window Behavior → Advanced → Placement: Maximizing, but it doesn't hide window header (which is about 20 px) and incorrectly maximizes some applications (like Emacs).
How to unfold every window to fullscreen automatically on launch in KDE?