No, the Aurora design system is not mandatory.
The Aurora design system was primarily built for use by the Digital Collaboration Division for the creation of the new Open Accessible Digital Workspace.
Because Aurora is an open source project, it is made publically available for any partners across the Government of Canada or elsewhere to use and learn from. Aurora, unlike the Web Experience Toolkit (WET), or the Canada.ca templates, is not mandatory for Government of Canada websites.
Aurora is meant to be a fun, easy to use system that will help guide the rebuild of the GCTools, and may provide useful for others looking to build interactive applications.