The current doc block description of 'ignoreExtraKeys' in Symfony\Component\Config\Definition\Builder\ArrayNodeDefinition is

"Allows extra config keys to be specified under an array without throwing an exception. Those config values are simply ignored. This should be used only in special cases where you want to send an entire configuration array through a special tree that processes only part of the array."

Could that be modified to include the sentence "and removes them from the resulting array".

It would also be nice if we could pass a boolean to the method to force it to leave those keys in place.