Consider deprecating old validation API

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

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

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

The issue mentions routes, validation, and error handling but names no files, tests, or entry points. Start by locating the route validation code and existing error messages; done means agreeing on the deprecation scope and improving the messages with coverage for the affected behavior.

Written by the indexing model from the issue text.