The window decoration named "KDE 2" is no more available in KDE 5. It was available in KDE 4 under kdeartwork package.
You are free to port it to the new kdecoration2 API or to an Aurorae theme, but KWin developers will not do it.
@Christoph Feck Are you aware of any kwin client decoration theme ported to KDE 5 to use as example?
A minimal example is at http://developer.kde.org/~cfeck/skeleton-0.1.tar.xz Both Oxygen and Breeze are fully ported to kdecoration2 API. The Plastik decoration is actually an Aurorae theme (those you can download via GHNS). https://cgit.kde.org/oxygen.git/tree/kdecoration https://cgit.kde.org/breeze.git/tree/kdecoration Aurorae guide at https://techbase.kde.org/User:Mgraesslin/Aurorae https://cgit.kde.org/kwin.git/tree/plugins/kdecorations/aurorae/themes/plastik
@Christoph Feck Thank you for the skeleton example, it's really useful! BTW do you know why 1px adjustements I make look correctly in systemsettings5 preview, but badly on a kwin? Perhaps because of subpixel anti-aliasing? How to disable it then? What I've tried in settings make no difference.
Okay, clearing of hints by adding painter->setRenderHints(QPainter::Antialiasing, false); to Decoration::paint() fixed issue