The feature is available on our Pro and Enterprise plans.
On qualifying plans, you can mark your track tags to be 'private' in Settings > Track Tags by clicking the lock toggle under 'Share Settings':
Once a tag is set to private, it won't write to the track metadata if you have the business setting enabled to write your track tags to the comments metadata field. Private tags also won't appear in Libraries.
Private tags are useful if you or your team want to use tags to designate something about tracks internally, but do not wish this information to be available to your clients.