The Insignia-class starship was created by 3D modeller and artist Mark Kingsnorth in 2008. He released his model to the public on December 20th, 2010.
To date, the Insignia-class has not appeared in canon, and has only been seen in the officially licensed Ships of the Line calendar series, and the books which collect those images. However, many fan-created projects make use of the class.
The USS Crucial was described as an Insignia-class starship through the use of photos and description in its bio on previous incarnations of the Star Trek: Phoenix-X website and the Star Trek: Crucial blog (both sites are now removed, with the former being updated). The ship as an Insignia-class was never mentioned in-story, but the Crucial was refitted into an Intrepid-class, from what can be surmised as the Insignia-class, in "For the World is Worldly and Such".
