Consider deprecating old handlers API

**Feature request**
It would be great to have serialization for models.

**Use case**
Many users need this functionality for production deployments.

**Implementation idea**
Add a new config option `enable_serialization` with default `false`.

The title discusses deprecating the old handlers API, while the body requests model serialization and an enable_serialization config option. Start by tracing the old handlers API and configuration entry points, then clarify which change is intended; done should include an agreed scope, default behavior, and coverage for the resulting serialization behavior.

Written by the indexing model from the issue text.