Good day.

How can I cut part of window and use it separatedly, using KWin?

For example, leave only part with some notifications opened and hide other part.

I don't really understand what you want to achieve. Could you explain this furhter?

You cannot slice windows and esp. not their contents with the WM.
Either the client supports hiding certain information or des-integrating GUI (like QMainWindow and QDockArea) or it's not possible.
The windowmanager sees every window as unit and has absolutely no idea about it's internal concepts.