Unfortunately when you create a new Plunker demo, the files are imported in a way from NPM to always take the latest version. This is good in a way and bad in another as now with the release of Angular version 6 & RxJS 6 most Plunkers simply break.
The best option is to upgrade and give your users/readers the latest and greatest stuff Angular version 6 and RxJS version 6 provide.
If you cannot upgrade immediately, please consider patching your config.js
in Plunker with the file below to make sure your users still have a functional and running demo 😃
Thx ❤️