What is the recommendation for mapping state? The configuration in D8 is nicely mapped in config schema file so any developer knows what configuration changes can be made. But what about state? Is there a recommended way to map state? Or is it the same situation as with variables in D7?

Would it be a good idea to store default values for state as part of the config schema under some specific key, like "state"? Althoguh that would mean dependency on config(factory) in different places within the code.
There is no mapping/schema for state nor for the underlying key value system.

This question can't be "answered" other than saying "None". Anything else would need to be in the form of a feature request and I'm not convinced this makes sense.