Playlist windows are back in iTunes 12.6. This is the feature in iTunes whereby an individual playlist can be opened in a separate window from the main iTunes browser window. There is a command in a playlist’s contextual menu to “Open in New Window”. It used to be you could double-click on a playlist to open it into a playlist window but this has not returned (Update, 3/22/2017: Command-double-click on the name of a Playlist in the Sidebar to open it in a Playlist window).
Playlist windows disappeared with iTunes 11 and at the time I suspected it had something to do with the way playlists were being re-designed for integration into iTunes Match/iCloud Music Library. I guess they figured it out.
The AppleScript property for playlist window has always been around but was ineffective once the actual UI element was removed. It still works. However, many of my scripts have been updated over the past few years to ignore playlist windows; perhaps there will be some opportunities to bring back this option. For the most part, scripts that require tracks to be selected will still only look for a selection in the browser window. One other thing: you can’t make a playlist window with AppleScript—something I’d always pined for—so you have to manually create the window. And, as Kirk points out, there’s no separate window for Store and For You and stuff of that ilk.
Still, nice to see playlist windows are back. Those of us who spend a lot of time tinkering with tracks really missed them.