Index: src/wp-includes/option.php
===================================================================
--- src/wp-includes/option.php	(revision 42637)
+++ src/wp-includes/option.php	(working copy)
@@ -2182,6 +2182,11 @@
 			remove_filter( "sanitize_option_{$option_name}", $wp_registered_settings[ $option_name ]['sanitize_callback'] );
 		}
 
+		// Remove the default filter if a default was provided during registration.
+		if ( array_key_exists( 'default', $wp_registered_settings[ $option_name ] ) ) {
+			remove_filter( "default_option_{$option_name}", 'filter_default_option', 10 );
+		}
+
 		unset( $wp_registered_settings[ $option_name ] );
 	}
 }
Index: tests/phpunit/tests/option/registration.php
===================================================================
--- tests/phpunit/tests/option/registration.php	(revision 42637)
+++ tests/phpunit/tests/option/registration.php	(working copy)
@@ -94,4 +94,19 @@
 	public function test_register_deprecated_group_privacy() {
 		register_setting( 'privacy', 'test_option' );
 	}
+
+	/**
+	 * @ticket 43207
+	 */
+	public function test_unregister_setting_removes_default() {
+		register_setting(
+			'test_group', 'test_default', array(
+				'default' => 'Fuck Cancer',
+			)
+		);
+
+		unregister_setting( 'test_group', 'test_default' );
+
+		$this->assertFalse( has_filter( 'default_option_test_default', 'filter_default_option' ) );
+	}
 }