Inuits' diet consists almost entirely of meat. As per my understanding, meat does not provide us with fibre and fibre is obtained from plant products only. Do the Inuits suffer from any malnutrition or digestion issues because of a lack of fibre in their diet? Have their digestive systems adapted to their diet or does the climate they live in have a role to play? If so, what would happen if an Inuit person changes to a "mainstream" diet that includes cereals and vegetables? What if a "mainstream" person goes vice versa?
Essential nutrient means any substance normally consumed as a constituent of food which is needed for growth and development and/or the maintenance of life and which cannot be synthesized in adequate amounts by the body (fao.org).
Dietary fiber does not meet the criteria of being needed for life/growth, so, strictly speaking, it is not an essential nutrient. Anyway, according to some authors "dietary fiber is essential for a healthy diet" (Mayo Clinic), for example, insoluble fiber may help maintain bowel regularity and soluble fiber may contribute to intestinal health and lower cholesterol levels. Fiber may also slow down digestion and thus help to control blood sugar levels (PubMed).
Inuits do not eat only meat but also plant foods, such as berries, tubers, roots, seaweed...(Wikipedia: Inuit Cuisine) and, according to the documentary The Last Ice Hunters, some commercial food delivered from Denmark.
Results of a 1930 experiment, in which 2 men were eating exclusively meat (no plant food and hence no fiber) for a year:
At the end of the year, the subjects were mentally alert, physically active, and showed no specific physical changes in any system of the body.
The control of the bowels was not disturbed...
...the clinical observations and laboratory studies gave no evidence that any ill effects had occurred...
From this very small experiment it is not possible to reliably predict long-term effects of lack of fiber in humans, though.
Conclusion: While not essential for life, fiber can be good for health, especially for bowel regularity.