3ED_0  filed a deletion request for gtk2-theme-slickness :
Last update has been 9 years ago. Gtk2 only. ---- https://www.gnome-look.org/content/show.php/SlicknesS?content=71993
Request #10598 has been rejected by Alad :
That's why it says "gtk2" in the name... if the theme is still functional there's no need for it to be removed.