Consider deprecating old lib API

**Problem**
When working with config, 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 configuration handling and the old library API in the repository; the issue names no files, entry points, or tests. Clarify which API is being deprecated, what compatibility period is expected, and which config errors need more detail. Done means an agreed deprecation path and descriptive error behavior, covered by relevant tests.

Written by the indexing model from the issue text.