I'm running plasma 5.1.95 and I notice that with the Breeze window decoration (which is the default) tabbed windows is broken. I get that most window decorations don't support this feature (it seems to be Oxygen or nothing), but we're not seriously going to see a release in which the default decoration breaks something so useful and important... are we?

I agree - 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.

Bump; this is a feature I use daily at work.