Maintenance is part of the design
A system is not finished when it ships. It is finished when another person can safely understand, repair, and eventually retire it.
Maintenance starts with small acts: meaningful names, logs that describe decisions, backups that have been restored at least once, and defaults that fail closed.
Leave a map
The most useful documentation describes boundaries. What is this component responsible for? What does it depend on? Which data is irreplaceable? A short, current map beats an exhaustive guide that nobody trusts.
Design the exit
Every system will one day be migrated, replaced, or shut down. Clear storage formats and reversible changes make that ending uneventful—which is exactly what good operations should feel like.