Originally Posted by
Pilot DAR
These are not the only criteria for possibly requiring an STC. Restraint means for occupants are a "major mod", and also must comply with TSO's. Restrain of items of mass in the cabin require approval. There would be responsibility for a pilot who allowed the use of "alternate" restraint means during their flight. I would be sure that approvals are correct for the aircraft and operation.
I'm aware those aren't the sole criteria. As the harnesses attach to existing points, don't replace existing equipment, and aren't legally required at any phase of flight, why would they require an STC? Seatbelts are still used for takeoff and landing. In the US, seatbelts are not required for passengers during flight.