deprecation-and-migration

规划与执行功能/系统弃用和用户迁移的策略与步骤。

规划并执行功能或系统的弃用（deprecation）与用户迁移（migration）策略，包含风险评估、分阶段下线、兼容性保证和沟通计划。在要移除旧 API、替换实现或决定是否维护遗留代码时使用，确保用户和依赖方平滑过渡。强调可回滚的迁移路径、明确时间表和文档化步骤，避免突发中断与长期技术债。

# 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