Exact namespace prefix and non-empty endpoint name helped me:
add_action('rest_api_init', function () {
register_rest_route('vku/notifications', 'test', [
'methods' => 'GET',
'callback' => function (WP_REST_Request $request) {
error_log('Route "notifications" works');
return new WP_REST_Response(true, 200);
},
]);
});