In this case, we don't have good data representations for code which produces or consumes structured data, where the structure of that data changes over time. We have some pieces figured out for certain domains, such as database schemas, but we do not have a "theory-of-everything" for enforcing types/structure across all domains of our systems. The domain addressed by GraphQL, HTTP APIs, is another instance of this same problem.

There are experimental systems like Unison that keep old versions of your code alive as data, which are fascinating solutions to the underlying problem.