Consider deprecating old middleware API

**Description**
Currently validation lacks proper notifications support.

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

**Priority**: Low/Medium

No files, tests, or entry points are named. Start by locating the middleware validation and existing notification handling, then clarify whether the intended work is deprecating the old API or adding configurable notifications; done means the agreed behavior and defaults are implemented and covered by relevant checks.

Written by the indexing model from the issue text.