Not true. You can absolutely get support from Apple while running a beta. Done it many times. Do you have anything to support this claim?
Yes. Its oficial policy that you won’t get Support for issues with your hardware when you are on Beta. When Apple was doing cheap battery Exchange the wouldn’t change my battery because I was on beta. They said there test Software wouldnt run. I was on 73% battery health.
Happened in official Apple Store in Stuttgart Germany