Note that KDE still ships at least 4 desktop themes on any reasonable distro, including "MS Windows 9x" that still has a similar look to these screenshots.
You can still use the KDE 3 and 4 themes (Plastik and Oxygen) throughout Plasma 5.
They're not exactly bug-free though. For example, when I enabled the Oxygen theme and adjusted the taskbar height to roughly where I had it in KDE 4, all the text became about three pixels high for no reason. I think there were other problems too.
You can still use the KDE 3 and 4 themes (Plastik and Oxygen) throughout Plasma 5.