5.0 shipped on 6 December with the new editor as the default, and the escape hatch shipped alongside it — which is unusual enough to be worth reading as a statement about how disruptive everyone expected this to be.
// the plugin is the supported route, not a filter
// wp plugin install classic-editor --activate
// per user, if the site should offer both
add_filter( 'classic_editor_network_default_settings', function ( $settings ) {
$settings['allow-users'] = true;
return $settings;
} );The plugin is maintained by the core team and the commitment at launch runs to the end of 2021, which is a long time in this ecosystem and long enough to plan a migration rather than react to one. Treating it as permanent is the mistake — every month it is installed is a month the site’s custom editing UI is not being rewritten, and the deadline does not move. Deciding the migration date at the same time as installing it is the difference between a plan and a deferral.