> When referring to "state" are we talking about just mutable state?
I don't think you should restrict yourself to thinking only about mutability and immutability in your program, but also that of the entire system. If your program is completely self-contained, that's good, but often they need to integrate with outside services and communicate over the network and write data to disk etc.
Those dependencies also result in state that might affect the behaviour of your software and you need to consider it when designing and writing code.
I don't think you should restrict yourself to thinking only about mutability and immutability in your program, but also that of the entire system. If your program is completely self-contained, that's good, but often they need to integrate with outside services and communicate over the network and write data to disk etc. Those dependencies also result in state that might affect the behaviour of your software and you need to consider it when designing and writing code.