Reading you it was as if Microsoft decided to maintain two web browsers at the same time (which would probably not be a good business strategy), so I checked on Wikipedia and found this:
Microsoft Edge, officially unveiled on January 21, 2015, has replaced Internet Explorer as the default browser on Windows 10. Internet Explorer is still installed in Windows 10 in order to maintain compatibility with older websites and intranet sites that require ActiveX and other Microsoft legacy web technologies.
According to Microsoft, development of new features for Internet Explorer has ceased. However, it will continue to be maintained as part of the support policy for the versions of Windows with which it is included.
I think this explains why they didn't bother testing Funimation on IE11, as we are supposed to be using Edge now.