SSD: Server Side Decorations
CSD: Client Side Decorations
It’s how kwin fundamentally works. And why you can’t just have application specific buttons and widgets in kwin window decorations like how in Gnome you can.
The article I linked is from 2013. They’ve been discussing this for a long time.
I imagine the code is not the problem, but more of a philosophical one. I am on your side that this is sorely needed in KDE. But I’ve been seeing KDE devs shoot down this type of functionality for a decade now and the state of this MR looks like more of the same.