New HTTP headers signal API

Short summary

Use HTTP Deprecation and Sunset headers to signal API endpoint retirement directly in responses, enabling clients to detect and react programmatically. Include migration documentation via Link header. Practical implementations in Express and FastAPI demonstrate automatic client-side warning detection, eliminating surprise breaking changes.

• •Deprecation and Sunset headers make API retirement self-documenting and machine-readable
• •Clients can automatically detect deprecated endpoints and warn developers without manual notification
• •Copy-paste middleware examples for Express and FastAPI with client-side detection wrapper

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