I'm the guy who writes the Twin Otter AFM.
Flapless landings are prohibited as a normal operational practice. There are a variety of reasons for this, high touchdown speed and inability to successfully carry out a baulked landing in accordance with the certification criteria when operating in that configuration are two of them. de Havilland did not "forget to approve the test", that is utter nonsense. De Havilland did not approve 0° flap landings because they do not provide the margin of safety required by CAR 3 and SFAR 23, the two certification criteria that are applicable to the Series 300 Twin Otter.
Supplement 23 to the Twin Otter Supplementary Operating Data (PSM 1-63-1) provides some guidance to operators when a Twin Otter must be landed with flaps retracted. The title of the supplement includes the very clear notation EMERGENCY OPERATIONS ONLY.
Only flap 20° and flap 37.5° (full flap) landings are approved as normal operations practice for Series 300 landplanes. Flap 10° landings are approved during icing conditions, or if there is any risk of ice being present on any part of the airframe when landing.