I have the impression that they have changed a couple of things to improve novice experience at the expense of configurability.
I'm not sure about what has been changed or when. But as for imputing a reason for the way it is now, the opposite is closer to the truth. Rather than files being arbitrarily restricted based on where they happen to reside, we're free to include any files in the library. That clearly supports configurability, while probably making things more challenging for novices who are likely to understand their file system better than the database.
Adding/removing a path should add/remove the content from the library
"Adding/removing a path" from where? Removing a path from an import specification should do nothing but stop future importing of files from that path. Removing files has nothing to do with importing. And while there's no separate command for it, there's nothing stopping you from selecting all files in a particular root path and removing them from the library.
Also, let's not forget most users would not choose to maintain multiple libraries. Doing so obviously involves more maintenance effort, so it should only be done when there is some compelling need that justifies the additional effort. Part of that would include the presence of mind to store media destined for different libraries in different locations, and to configure each library to import only it's own media. Most users, however, are better served by maintaining all media in one library—relying on the flexibility of the program to have it all handled appropriately. For example, there's no reason why popular and classical music can't be effectively handled in the same library—just as there's no reason why music, images and video can't be effectively handled in the same library.