Website can't work with encrypted data without the key

In EncryptionMethodInterface, this is the phpdoc for the decrypt method:

  /**
   * Decrypt text.
   *
   * @param string $text
   *   The text to be decrypted.
   * @param string $key
   *   The key to decrypt the text with.
   *
   * @return string
   *   The decrypted text
   *
   * @throws \Drupal\encrypt\Exception\EncryptException
   *   Thrown when decryption fails.
   * @throws \Drupal\encrypt\Exception\EncryptionMethodCanNotDecryptException
   *   The method should throw this exception when the plugin can not decrypt
   *   (i.e. use a public key).
   */
  public function decrypt($text, $key);

According to the interface, we have to return the decrypted text. So if you want to change this behavior, you need to make a feature request upstream with the Encrypt module. Once the interface is changed, then we can change the behavior here.