With the release of iTunes 12.6 on macOS Sierra, Apple restored the ability to open a playlist in a new window. Why this feature was ever removed shall forever remain a mystery. But here's how you can open a playlist in a new window.
- Launch iTunes.
- Control-click or right-click on the playlist you want to open and select Open in New Window.
Voila! That's it.
(This how-to is based on my experiences and info on Apple's support pages — where the images sometimes come from.)