Thanks to all have responded, and Steven, thank you for taking the time to review this request.
Over the past couple years, several use cases have come up where this feature would be incredibly useful. A few examples:
1. A radio station uses MusicBee for certain production purposes and wants to limit the Library to a specific set of files at any given time, fed through automation into 3 specific folders and synced routinely. The station personnel only have time to feed the folders and do not have time to adjust the MusicBee library manually.
2. An online music reviewer deals with tens of thousands of albums and wants to keep various instances of albums (various pressings, versions, etc.) in the MusicBee Library at certain times via many monitored folders, using MusicBee functionality for various file processing and ease-of-review purposes, but then needs to completely clear out these Library entries periodically with folder removal. An automated process as described would help immensely; the current steps required are too manual.
3. Many users work to refine their audio collections over time and prefer to keep a "gold standard" set of folders which define the "correct" versions of the collections. Sometimes work is done on those folders which can sometimes result in unwanted duplicates or alternate versions in the Library, and sometimes the results are too messy to deal with manually. An automated sync-Library-to-Monitored-Folders feature would resolve this challenge.
There are many more cases that could be listed.
Responding to items noted above:
One potential issue I see with files being removed automatically is for files on external drives.
If the drive is disconnected MusicBee might wipe the library clean.
True. Perhaps MusicBee could recognize whether the drive itself is available to the system or not, and only take action per this feature if (a) the drive becomes accessible again and (b) indeed a Monitored Folder has been changed.
As long as it only removes them from the library and doesn't delete them, I don't see a problem with it as an optional setting, although it would probably be wise to offer a confirmation dialogue similar to the current one about removing missing files.
The confirmation dialogue sounds like a good idea.
Thanks again for your consideration of this feature. It would truly be helpful in a wide range of situations.