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I don't know how X/KWin session are working, so I don't know is that able to realize.

The idea is to save tab group of KWin to icon and automatically close application in that window with session save. Additionaly user will be prompted to select location of icon to restore closed window group.

So, this feature to close the applications in a tab group and save a shortcut to relaunch all applications in the group; Is it a button/menu item on the tab group which can be selected at any time, creating an icon which can be launched at any time? Or is it an option when logging out (or saving/switching desktop session) to save out any tab groups so that they can be optionally relaunched next time the session is started?

The first option to add a button should be pretty feasible. The second option needs a bit more explanation as to what exactly you wish the feature to accomplish.
As I understand it, kwin should automatically restore things like window locations including tab groupings when restoring a saved session (and if it doesn't remember tab groupings, that's probably a feature that should be pretty easy to implement), so I'm not sure what more you expect here.

There should be button on titlebar, which closes all application in that window(window group) and save session of that application and allows to create icon to relaunch all applications, which were in saved window. KDE supports activities, so it splits session management between activities, so my idea should be able to realize.

Of course, this option should been disabled, when non-kde application reside in window. KDE shouldn't close application, but only windolws, because there can be many windows in many activities/windows for each application.