From the documentation:
Enable the API in insecure mode, which means that the API will be available directly on the EntryPoint named "traefik". Note: If the EntryPoint named traefik is not configured, it will be automatically created on port 8080.
I tried to set its value to false to see if there was any effect, but none that I could see: I was still able to make request to the /api.
So what does it do exactly?