While atomic big-bang releases have their benefits (and drawbacks), I don't think there's any way to avoid dealing with data backward compatibility in at least

While atomic big-bang releases have their benefits (and drawbacks), I don't think there's any way to avoid dealing with data backward compatibility in at least some way. Old versions of data always exist in your system (in databases, message queues, replay logs, caches, retry loops in external parties, even on the wire at the time of the release in some cases). While explicit and long term API versioning may not be needed in some release processes, a strategy for coping with old data becomes necessary past a certain scale. Migrating all extant data (not just your RDBMS) at the same time as the big-bang release is not practical.