Consider deprecating old auth API

**Description**
Currently config lacks proper serialization support.

**Proposed solution**
Add configurable serialization with sensible defaults.

**Priority**: Low/Medium

Start by locating the authentication API and configuration serialization entry points; the issue names no files or tests. First reconcile the title's deprecation request with the body’s serialization proposal, then define the affected configuration formats, defaults, and tests before implementation.

Written by the indexing model from the issue text.