It's not public, but it's worth deprecation process, because some people may rely on this method. When the warning should be emitted? I think that it should happen if `validate_field` method is overriden. If this is the case, there is an issue -- how can we check if the method is overriden? We can delete the default `validate_field` method and `check_field` will check if `validate_field` is defined and if this is the case, the method will be called and there will be a warning emitted.