KDE3’s Plastik style was closer to XP’s Luna than Vista/7’s Aero. The default icon theme CrystalSVG had a colorful pastel and cartoonish style. There’s a full port of it to Plasma 5 here. It should still work fine in Plasma 6.

The Plasma 4 Oxygen style was much closer to the detailed realistic skeuomorphic style of Aero (though it was probably more so inspired by early OSX Aqua). It is still usable on Plasma 6 and packages for the theme should be available in your preferred distributions’ repositories.

Crystal Remix is really a mix of three icon themes designed by Everaldo Coelho in the 2000s. Only the first of these, the aforementioned CrystalSVG, was ever an official KDE theme. The second is Crystal Clear, the successor to CrystalSVG. It had a more detailed and realistic style compared to CrystalSVG, looking far closer to Aero. The third one is Crystal Project, the final iteration of Crystal. It went further into Crystal Clear’s direction and erased the last vestiges of CrystalSVG’s more cartoonish style. Crystal Project’s icons are particularly detailed and I’d consider it a really underappreciated piece of skeuomorphic icon design.

Personally, I’m not that big on Crystal Remix. I’d prefer either a complete CrystalSVG-styled icon theme or a complete Crystal Project-styled icon theme. Preferably the latter because the former already pretty much exists. The two styles don’t mesh together all that well, IMO. Crystal Remix’s coverage (especially for action icons) is also a bit lacking and it’s pretty common to run into unthemed Breeze icons in some applications.

Crystal Dock doesn’t strike me as particularly KDE3-ish. It’s basically nothing like KDE3’s Kicker, though there were a lot of popular third party docks in the KDE3 era like KoolDock, which were actually quite similar to this project. All of them are gone by now and there’s been a lot of demand for new third party docks since the death of Latte Dock. So I anticipate this project will make quite a few people happy.