As the Micropub spec has evolved, there have been some minor changes to how Quill used to do things. Today I added a configuration option to allow you to change the name of the property used to indicate the syndication targets.
Quill used to always send the syndicate-to property, but now the recommendation is to use mp- prefix to indicate Micropub commands rather than properties of the post.
This backwards compatibility section in Quill allows me to make changes to the client without affecting existing users until they're ready to opt in to the change. All existing Quill users will still be receiving the syndicate-to property until they go into the settings and change it to mp-syndicate-to. However, new users will default to the new mp-syndicate-to property.