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 but everybody depends on
• Planning the lifecycle of a new system (deprecation planning starts at design time)
• Deciding whether to maintain a legacy system or invest in migration

Core Principles

Code Is a Liability

Every line of code has ongoing cost: it needs tests, documentation, security patches, dependency updates, and mental overhead for anyone working nearby. The value of code is the functionality it provides, not the code itself. When the same functionality can be provided wi…