Where's the context menu option to tab windows together in 4.14? It wasn't taken out, was it?

works for me, it might only be available for certain window decorations iirc so try switching to oxygen - can't recall if it will work with others

Yeah... that was it. Tht really kinda sucks...

work for me guys, there is no problem

Yes, it's decoration dependent - Oxygen and (there used to be another that I can't remember) - I'm just starting to test plasma5 now, and losing tabbed windows would be huge regression.
I've not been active in the community - where is the most effective place to vote/request/whatever to get tabbing support either added to the breeze decoration, or more usefully enabled in kwin regardless of decoration?
I don't know if any distros turn on 'Group similar' by default, but tabbed windows are mind-blowingly great to anyone who finds them.