deprecation-and-migration

管理功能弃用与迁移流程，降低维护成本与破坏面。

把功能弃用和迁移当成工程化流程来管理，包含通知、兼容策略、迁移步骤与回滚方案，目的是降低维护成本和破坏面。会在要下线旧 API、合并功能或把用户迁到新实现时使用。强调渐进迁移、可观测与阶段性回滚，确保体验平滑且风险可控。

# Deprecation and Migration

## Overview

Code is a liability, not an asset. Every line of code has ongoing maintenance cost — bugs to fix, dependencies to update, security patches to apply, and new engineers to onboard. Deprecation is the discipline of removing code that no longer earns its keep, and migration is the process of moving users safely from the old to the new.

Most engineering organizations are good at building things. Few are good at removing them. This skill addresses that gap.

## When to Use

- Replacing an old system, API, or library with a new one
- Sunsetting a feature that's no longer needed
- Consolidating duplicate implementations
- Removing dead code that nobody owns