deprecation-and-migrationlisted

# Deprecation and migration ## Decision questions 1. Does the old system still provide unique value? 2. How many consumers? Migration cost each? 3. Is replacement production-proven? 4. Cost of NOT deprecating (security, maintenance)? ## Compulsory vs advisory | Type | When | |------|------| | Advisory | Stable legacy, optional migration | | Compulsory | Security risk, blocks progress — need tooling + deadline | Default advisory unless risk justifies force. ## Process 1. **Build replacement** first 2. **Announce** — status, replacement, migration guide, timeline 3. **Migrate consumers** incrementally; owner migrates users when possible 4. **Remove** only after zero active usage verified ## Patterns | Pattern | Use when | |---------|----------| | Strangler | Gradual traffic shift | | Adapter | Old interface, new implementation | | Feature flag | Per-consumer or cohort cutover | ## Zombie code signals - No maintainer, active dependents, failing tests ignored, stale deps ## Output Deprecation notice + migration steps + removal checklist.