The method \Magento\Framework\App\Config\ValueInterface::isValueChanged() should be removed from the interface because it is used only internally by a particular interface implementation: \Magento\Framework\App\Config\Value

dmitrii_fediuk (Dmitrii Fediuk) January 11, 2016, 12:15pm 2

See also:

The methods \Magento\Framework\App\Config\ValueInterface::getFieldsetDataValue() and \Magento\Framework\App\Config\Value::getFieldsetDataValue() are declared but never used
Logical error: Magento 2 checks whether a backend config form field’s «backend_model» implements the \Magento\Framework\App\Config\ValueInterface interface only, but actually calls the «backend_model» for much more methods (absent in the interface)
The method \Magento\Framework\App\Config\ValueInterface::getOldValue() should be removed from the interface because it is used only internally by a particular interface implementation: \Magento\Framework\App\Config\Value
A backend model should always be a \Magento\Framework\Model\AbstractModel