> However, if you just want a very minimal subset of a JSON response (i.e. a 'status' string field out of say 20 other values in a map), is it not better to just look for one field, as opposed to have to create an entire struct in Rust to represent the full response?

You can define a struct with just the status field.

> How does Serde work with optional extra fields? I know you can use Option<>, but that implies you know they exist - what happens if fields get added in the future silently? I guess the schema changes then, which isn't good, but that might happen?

Unknown fields are skipped by default.