Sniff deprecated deps

Clear and concise description of the problem

We don't suggest anything to the user rn

Suggested solution

Renovate does a decent job here:

Alternative

No response

Additional context

No response

Validations

• Follow our Code of Conduct
• Read the Contributing Guide.
• Check that there isn't already an issue that request the same feature to avoid creating a duplicate.

Already implemented — closing as resolved.

src/services/deprecated-dependencies-checker.ts queries npm and Packagist deprecation flags (including any suggested replacement), and the dependency dashboard renders a "Deprecated dependencies" section from it.

Since this issue was filed, deprecation has also become a merge-gate signal: gates.preMerge.dependencyGate with blockDeprecated will fail a pull request that introduces a deprecated package (#1382), and that check runs with no API key.