I know that, the question is why. The discussion started with a user asking for talks about Plasma going with Client Side Decorations in the future. To which I say: No.

A header bar is the logical conclusion of CSD, because it makes sense once you give window decoration responsibilities to app devs. That is space that the dev can use to cram buttons and other features, why wouldn’t they use it?

While KDE could create documentation that suggest using CSD in a way that looks similar to the current title bars, that is more what you’d call guidelines than actual rules. It is not the great equalizer that SSD is. If an app uses SSD, there is no question on whether the Title Bar will look good, it will look the way it is supposed to, no wondering if the developer implemented the design in the correct manner. With CSD, the dev could not have followed KDE’s guidelines, and even then, there is the question of what to do when you are not in Plasma. Should an app made with KDE Guidelines in mind make changes to its CSD when it runs on GNOME? Some GNOME apps don’t, should it be mutual?

And that is not even to mention the idea of using CSD to look like it is SSD. Again, you are trusting the app dev to make something that is currently taken care by the system. It doesn’t really makes any sense, you are adding another point of failure.