IE11 is supported by default if you use ReDoc as a standalone package.
If you use ReDoc as a React component you should include the following polyfills in your project:
import 'core-js/es6/promise';
import 'core-js/fn/array/find';
import 'core-js/fn/object/assign';
import 'core-js/fn/string/ends-with';
import 'core-js/fn/string/starts-with';
import 'core-js/es6/map';
import 'core-js/es6/symbol';
import 'unfetch/polyfill/index'; // or any other fetch polyfill
import 'url-polyfill';