HTML, CSS, and JavaScript code that was only in place to support Internet Explorer 6, Internet Explorer 7 and Internet Explorer 8 has been removed from Drupal 8 core.

Usage statistics for IE6, IE7 and IE8 continue to drop (statcounter, sitepoint, w3counter, theie7countdown) and are forecast to be even lower when Drupal 8 is released.

Removing full front-end support for IE6, IE7 and IE8 also has many benefits:

Allows for more complex and/or efficient CSS and JavaScript to be written e.g. IE7 does not meet ACID2 test.

Many features are not available to IE8 and under: caniuse.com: IE 6,7,8,9

Reduces the amount of code delivered to browsers on each request, potentially improving performance in all browsers e.g. html5please. Core now ships with jQuery 2

Frees up time previously spent developing and testing IE6 and IE7 fixes, allowing core developers and contributors to work on other tasks.

A module or theme can add back IE6 or IE7 compatibility if desired, but core will no longer offer any specific IE6 or IE7 front-end fixes (CSS and JavaScript). Some code supporting these browsers still remains in core (e.g. Form API) to ensure basic functionality.

JavaScript can break in IE6/7/8 and core does not take special measures to avoid it.