Form option cascade_validation has been removed in 3.0 (17cedd3), but FormValidator still contains code for handling it: https://github.com/symfony/symfony/blob/master/src/Symfony/Component/Form/Extension/Validator/Constraints/FormValidator.php#L141

I think this can now be removed .
And (I guess) allowDataWalking() method should return false by default.

Should I post a PR?