Applying a default sensitivity label to a SharePoint Online document library is just one of the set of security and management and governance features requiring the new Syntex Advanced Management license. The new license is in preview so all the features that it covers might not be fully baked. Microsoft 365 customers might well ask if this is yet another example of Microsoft bundling features into a new paid-for add-on license. Of course it is. You don’t expect new functionality for free, do you?