Not all parts of the code is equal in this respect. Those parts pertaining to the user visible portion (API of a library, command args of a CLI, UI of a GUI/TUI app, endpoints in a web service, etc.) are closely related to the spec. The rest is more fluid as long as it does not change user visible behavior. The choices still affect maintenance and debugging costs, so there is some pressure to not YOLO these portions. I think the most difficult design decisions relate to how to separate the two and how to ensure a smooth evolution of both user facing and programmer facing design decisions.

What is different now is that maintainability and debugging design decisions were made w.r.t. human coders or teams in the past which is not necessarily the case anymore. Should we just specify the API and let agents figure the rest or do we still want to control the rest to ensure maintenance and security? A year ago I definitely thought so. Now it is more murky as the agents are faster browsers of codebases and can explore runtime effects faster than I can type and parse output. Strongest empirical observations depend on the runtime behavior so they have an edge there.