The app I work on has to support IE11 and it's easily the worst part of the job. Often times regressions are made because someone accidentally used some js keyword/feature that's not supported. The build takes forever because of IE, and so there is a flag IE_BUILD that's false most of the times until someone has to test in IE at which point they turn it on and wait for 10 mins for the thing to build. We had to implement theming, and for every other browser we used CSS variables but for IE we had to implement a hacky/complicated solution where we compiled customer specific CSS files.
In short, IE sucks and I wish it'd die.