Currently, if you upgrade forms (or other parts) using the following hook:

protected function _buildValidator(Validator $validator): Validator {}

You will now silently have a passing form (no validation anymore at all) because the callback is never executed.

We need to document this in the guide for those cases - or add dummy hooks that trigger deprecation/exception.

It is now:

public function validationDefault(Validator $validator): Validator {}