The Playlist Editor is still there. Multiple Playlists in Winamp 5.X are in the Media Library under Playlists. You can access them by the Manage Playlists button in the Modern Skin or by right clicking on the LIST OPTS button in the Classis Skins.
(Winamp3 1.0 was a separate product from Winamp 2.X. Winamp 5.X is a continuation from Winamp 2.X, therefore nothing was removed).