There will never be Client Side Window Decorations in KDE Plasma. In fact the current state of Wayland support in KWin requires server side Window Decoration for Wayland Clients.
We will most likely go for an approach were all Wayland Clients will be forced into server side decorations no matter whether they use Client Side Decorations or not. We have to do that as the differentiation between our Workspaces (Desktop, Netbook, Tablet) is from Window Manager perspective mostly in the decorations (Desktop always decoration, Netbook decorated except maximized windows, Tablet undecorated). I don't expect that application developers will care about such differentiation and we need to have the possibility to experiment in that area. We would limit ourselves if we would allow Client Side Decorations.
I have even considered to enforce server side decorations on X11 but that's difficult due to backwards compatibility.