Incomplete migrations are technical debt

When moving from one version of a library or service to another, or when switching libraries entirely, clean up the old version/dependency before declaring victory. Accreting multiple versions in your codebase generates complexity and bugs -- aka technical debt.

If the old APIs, RPC endpoints, etc remain, users will continue to use them, even if it's not the right thing to do.

See also Hyrum’s Law and Turndowns, not just deprecations