Architecture & practice·Legacy systems and migration
the replacement went live a year ago and the old service is still running because nobody proved it was idle.
Sunsetting
Also calleddecommissioning, end of life
The deliberate process of turning something off: announcing it, migrating remaining users, and deleting it. It is the step that converts a migration from a cost into a saving, and it is routinely skipped because it delivers nothing visible. The technique that actually works is to make the old system prove it is unused — turn it off for an hour, watch what screams — rather than to wait for certainty that will never arrive.