No matter how many reminders you send about preferred protocols, users will always go the path of least resistance. That’s a particular problem when it comes to adding new files to SharePoint. Saving new files in synced Windows folders is not ideal or sustainable, but it is convenient for users. Unfortunately, this method does not use valuable metadata to help you organize and maintain the information.
A suitable compromise is to allow users to keep stuffing files in folders while incorporating metadata information through column default value settings. If the folder structure is logical and matches the desired metadata, this setting allows you to define a default value to a column based on the folder it is in.
To set this up, navigate to Policies > Settings > Column default value settings. Your document library folders will display on the left. Click each one to set the desired default value.