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?
Best Answer
Go to System Settings → Window Behaviour → Window Rules. Click on the ‘New…’ button. Add a description like ‘maximise everything on start’. Go to the ‘Size & Position’ tab, then enable the checkboxes for ‘Maximised horizontally’ and ‘Maximised vertically’. For each of them, set the drop-down to ‘Apply Initially’, and the radio button to ‘Yes’. Click ‘OK’, then ‘Apply’.
Now all windows will open maximised.
This includes windows you may not have thought of like docks, transient windows, dialogue boxes etc. You can exclude any of them by editing the rule you just made: go to the ‘Window matching’ tab for the rule, and unselect any window types you don't want affected. Try leaving just ‘Normal Window’.
The rule mechanism is very versatile. You can also modify the settings on a window-by-window or application-by-application basis, just click on a window titlebar's top-left icon, then go to Advanced → Special Window Settings or Special Application Settings. ‘Window’ makes a rule (or edits the existing one) for just that window, ‘Application’ for every window created by that application.
Update: you can also set the fullscreen status and/or remove window decorations with this mechanism. The window manager is your oyster.