Until Kubuntu 15.04, there was a very usefull function which let you adding multiple windows as tabs.
In Kubuntu 15.04 and 15.10 I didn't find this option anymore. Is it removed or hidden? How can I enable it?
Window tabbing has not been implemented in Plasma 5.x as yet, so you won't be able to tab windows.
Also please note that even if implemented, previously it was only available with the Oxygen window decorator, and it remains to be seen if it will only be available with Breeze.
As for when this will happen, I really don't know.