Consider deprecating old routes API

**Problem**
When working with validation, the error messages could be more descriptive.

**Suggestion**
Improve error handling and add detailed error messages.

**Context**
This would help with debugging and maintenance.

Start by locating the old routes API and the validation error-handling path referenced in the issue. Determine which routes and error messages are in scope, then identify existing tests covering them. Done means the deprecation and more descriptive validation errors are clearly defined and verified by tests.

Written by the indexing model from the issue text.