I've effectively stopped supporting IE11 for over 2 years now... Every other browser added async function support by the end of April, 2017, nearly 3 years ago. That's where I cutoff.
IE11 shouldn't be supported at this point imho, and I will not make efforts to do so. Supporting IE11 adds complexity and/or bundle sizes that are excessive to support what aren't even that modern of features at this point.
https://gist.github.com/tracker1/8eea53ec700a0fea725123516a30a70a