IE 11 is still supported by Microsoft, but not for long. Adding a development focus on IE in 2019 is a waste of efforts, unless it's absolutely necessary. You can do it in a progressive way, where a site is usable on IE, but won't support all the features.
Falling back to XHR can be as simple as checking if (!window.fetch).
There used to be some specific HTML comments for detecting if browser is IE (not supported in IE 11). But feature detection is a better alternative than browser detection.
Alright didn't know that. Feature detection there is then.
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