There are bugs in IE 11 (which has 5.88% usage on desktop according to NetMarketshare and is still supported by MS) - dropdown buttons and other interactions do not work due to JS errors:
SCRIPT1053: Const must be initialized
SCRIPT1014: Invalid character
SCRIPT1010: Expected identifier
SCRIPT1003: Expected ':'
How much of those IE users are also users of GitHub?
Shipping more modern EcmaScript versions than what's supported by IE has lots of advantages for users whose browsers can support it, so I'd hate it if the IE support came with the expense of everyone else's experience. There are ways to serve different JS bundles for different browsers, of course, but that comes with a maintenance cost for them.
Weird though that some buttons wouldn't work at all, because at least most of the basic functionality seems to work fine with JS disabled even. Maybe they should just disable JavaScript altogether for IE and it'd work better. That should be easy to implement too.
