I use pinned (always above + on all workspace/desktop) quite often.

And to make it ergonomic I scripted kwin and set some shortcuts.

So yes, you can have any window PiP the way you like. But it requires you to do a long sequence of actions. Versus a single click for very specific PiP behavior.

Consider a window in a web browser tab. You could click the PiP button, which will pop out a tiny window, most likely already in a corner of the screen. This window is a mini video player. Your original browser tab stays untoucher, still at the same place in your web browser tab list, the rest of the tab still readable and scrollable etc etc.

Or, you could clone the tab. Move it to its own window. Locate the video. Put it in full screen. Un-fullscreen the window. Click on the pin button. Resize the window to the corner.

Same result, but not the same effort.