REDIS_URL is only available in the apicast-* deployment configs. But it is also used by system-* components.
The suggestion is to add REDIS_URL to the base_env OR to all system DCs, so that the customers can replace the built-in Redis with their own.

BACKEND_REDIS_URL is also used by system-* components, however it's not reflected in the template. It's worth adding it to the template too.