Consider deprecating old handlers API

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

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

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

No files, tests, or entry points are named. First reconcile the title about deprecating the old handlers API with the body’s serialization request, then locate the handlers API and configuration parsing. Done requires a decided deprecation plan and an agreed `enable_serialization` behavior, including its default.

Written by the indexing model from the issue text.