redirect admin interface from default to classic style

client/my-sites/site-settings/settings-writing/main.jsx

@@ -1,4 +1,5 @@
 import { localize } from 'i18n-calypso';
+import { useEffect } from 'react';
 import { connect } from 'react-redux';
 import DocumentHead from 'calypso/components/data/document-head';
 import Main from 'calypso/components/main';
@@ -7,22 +8,32 @@ import WritingForm from 'calypso/my-sites/site-settings/form-writing';
 import JetpackDevModeNotice from 'calypso/my-sites/site-settings/jetpack-dev-mode-notice';
 import SiteSettingsNavigation from 'calypso/my-sites/site-settings/navigation';
 import { getSelectedSite } from 'calypso/state/ui/selectors';
+import { useSiteInterfaceMutationWithNotices } from '../site-admin-interface/use-select-interface-mutation';
 
-const SiteSettingsWriting = ( { site, translate } ) => (
-	<Main className="settings-writing site-settings">
-		<DocumentHead title={ translate( 'Writing Settings' ) } />
-		<JetpackDevModeNotice />
-		<NavigationHeader
-			screenOptionsTab="options-writing.php"
-			navigationItems={ [] }
-			title={ translate( 'Writing Settings' ) }
-			subtitle={ translate( "Manage settings related to your site's content." ) }
-		/>
+const SiteSettingsWriting = ( { site, translate } ) => {
+	const { setSiteInterface } = useSiteInterfaceMutationWithNotices( site.ID );
 
-		<SiteSettingsNavigation site={ site } section="writing" />
-		<WritingForm />
-	</Main>
-);
+	useEffect( () => {
+		// if feature/experiment is enabled
+		setSiteInterface( 'wp-admin' );
+	}, [] );
+
+	return (
+		<Main className="settings-writing site-settings">
+			<DocumentHead title={ translate( 'Writing Settings' ) } />
+			<JetpackDevModeNotice />
+			<NavigationHeader
+				screenOptionsTab="options-writing.php"
+				navigationItems={ [] }
+				title={ translate( 'Writing Settings' ) }
+				subtitle={ translate( "Manage settings related to your site's content." ) }
+			/>
+
+			<SiteSettingsNavigation site={ site } section="writing" />
+			<WritingForm />
+		</Main>
+	);
+};
 
 export default connect( ( state ) => ( {
 	site: getSelectedSite( state ),