Does MobX work with Internet Explorer 11
No, MobX 5 does not work with Internet Explorer 11, because MobX 5 use Proxy and requires ES6 compliant browsers. However, Microsoft Edge browser can work with MobX 5. See this chart for reference to see which browser supports the Proxy feature of ES6. You see that it is red for IE11 and green for Edge. You can also see MDN about what Proxy is and which browsers support it.
If you really need to support IE11, use MobX 4 instead. MobX 4 can run on any ES5 compliant browsers. And from this table, most browsers are now ES5 comliant (including Internet Explorer 11). Which version of MobX 4 should is out there that I can use? See version history from npm here.