We've heard statement many times about various aftermarket products but it's always been an "I heard that...". In 20+ years of being car seat technicians, no one has shared an account of it actually happening to them. So we did some investigating.
We did some searching online and couldn't find any verification of this statement. We reviewed our current policy and past policy and couldn't find it in any of the small print. We  reached out to agents and claims managers in multiple insurance companies. They said they never heard of claims being denied because an after-market product was being used whether or not it was altering the vehicle or its safety systems. We also reached out to a personal injury lawyer who said most products are unregulated and he had not heard of a single claim being denied for that reason.
None of the customers who reached out to us after experiencing a crash while using the Tummy Shield said anything about their insurance company not paying their claims.