Subscenes are still in the preview stage, and they recently even removed the ECS stuff from the package manager listings (you have to find the names of the packages directly from the forums) to prevent people from thinking that they were production ready.

I was using the ECS for the last 2 years, and I wouldn't recommend it to anyone who actually wants to get something into production. My guess is it will be somewhere late 2021 or 2022 before it's actually production ready.