Titlebars are a total waste of space, especially on maximised windows. Here is, in my opinion, the best way to get rid of them.
Hiding window borders (including titlebars) is easily done on Plasma on a per-window basis using the Window Operations Menu, default shortcut: Alt+F3, or on a per-application basis, by setting Window Rules. But that is quite slow and cumbersome, as you would have to do it manually for every window. Not good. You can also set a keyboard shortcut to hide and show window borders. Better, but you still have to do it manually.
Fortunately, there also exists a hidden setting in KWin to automatically hide titlebars and window borders on maximised windows. To apply it, edit the following file:
Find the section
[Windows] and add
BorderlessMaximizedWindows=true under it. It should look something like this:
Log out and log back in. Open any application and maximise it to see the effect.
This way you have the best of both worlds: floating windows still have titlebars and fancy shadow borders and whatnot, and maximised windows have none of that clutter, letting you utilise your screen real-estate to the fullest.