Gnome and KDE have both embraced flat designs but the Enlightenment community seems to still do some rather "original" decorations [1]. They do look rather retro these days though.
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.
It's under active development, yes. It has seen a major rewrite of a lot of foundation libraries after 0.16.
The artwork is not of the same quality, I guess, probably because a lot of artists have moved on, but it's a very solid environment, and the libraries that have been developed as part of the project are great. They're used in Tizen: https://www.tizen.org/events/presentations/tizen-native-disp... ; Carsten Haitzler (rasterman) has been working at Samsung for a few years now.
(Or at least he had been working for a few years when I read the presentation; I think he still does, but I'm not sure)
https://exchange.enlightenment.org/themeGroup/show/24