Wasn't this a feature in the Fluency Admin plugin? I hated that feature and that is why I never used that plugin. I know there are plenty of people who do use that plugin, but it really is annoying. I like how the collapsible menu is currently. It was a great feature when it was added and still is today. I am just not keen on adding drop downs/ flyouts in the WordPress admin UI. It is a little inconvenient and touchy to use. I mentioned this on twitter a couple weeks ago when I was playing around in the beta.

This feature is definitely a step back, and not a step forward. Maybe if there is a demand for this... why not add it as an option for users, like the blue and default colors? *shrugs*