Hi!
I think it’ll be a good idea to be able to configure an optional “available_state” parameter to every entity.
Let me explain the main use case with an example:
In my case I have a lot of devices connected through smart sockets, for example my TV or my Satellite receptor, so when I don’t use those devices, I can (automatically 
) turn off them to avoid the stand by consumption.
The problem is that normally all the integrations that are using Polling, will still continue polling the device, and in every attempt, it will write an error to the logs.
I think being able to configure an “available_state” would be a good solution for this cases, because I would be able to configure the available_state as the state of their corresponding smart switch, and home assistant will not try to do polling to a device that it can know that is turned off.
… or maybe the problem is that I don’t know how to sort this problem, so I’ll appreciate a lot your ideas
Thank you very much!