It was inevitable Microsoft would have to wipe the slate clean when it came to building a replacement for Internet Explorer. While IE 11 is a far cry from the creaky ship that was 6, there's just too much cruft to work with. Hence the creation of IE's replacement, "Spartan", or Microsoft Edge as it's now called. So, what's Redmond getting rid of? All the bad stuff.
Over on Microsoft's official Windows blog, Edge developers Charles Morris and Jacob Rossi have outlined some of the legacy components Edge will leave behind. Probably the biggest thing to get the axe is support for ActiveX. Before HTML5, or even Flash were big, there was ActiveX to make web pages more functional than text and images.
All "quirks" and compatibility hacks for old IE versions will be abandoned entirely. As the blog post explains, you'll have no choice but to use the accurately-named "Enterprise Mode" if you want older websites to render correctly.
Something the team hasn't been able to kill off is vendor prefixes. These are used in CSS to tell specific browsers how to display a webpage element. For example, Google Chrome uses "-webkit", while Microsoft has "-ms" and Firefox "-moz". Unfortunately, too much of the web is reliant on these, so Edge will support them "for the time being".
There are a bunch of other bits and pieces Microsoft is removing — for the nitty-gritty, hit up the official post.