Consider deprecating old middleware API

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

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

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

No file, test, or entry point is identified, and the title conflicts with the request for validation serialization. First locate the middleware API and the validation or configuration paths, then clarify whether the goal is deprecation or a new enable_serialization option; done should be defined by the agreed scope and tests for the resulting behavior.

Written by the indexing model from the issue text.