Microsoft kept supporting IE11 in Windows 10 on PCs due to the fact that many businesses still use web-based applications that support the browser. It also allowed Edge users to open up IE11-based sites in Enterprise Mode. In a blog post, Microsoft detailed how they plan to improve opening IE11 sites in Edge:
Using the Enterprise Mode Site List, you can dictate which sites Microsoft Edge should open in Internet Explorer 11. For those sites, Microsoft Edge currently shows an interstitial page that states "This website needs Internet Explorer 11" and automatically opens IE11. We received feedback from customers that the interstitial page is sometimes confusing for users, and the right browser should just open automatically. We've heard your feedback and, beginning with the Windows 10 Anniversary Update, the interstitial page will be removed by default. IE11 will simply load on top of Microsoft Edge with no extra visual affordances, helping your users more quickly complete the tasks they had set out to do.
The Windows 10 Anniversary Update will also allow businesses to just open IE11 sites with the applications they need to use, and open all other sites in Edge:
Starting with the Windows 10 Anniversary Update, we are introducing a new Internet Explorer group policy to restrict IE11 usage to only sites on the Enterprise Mode Site List: "Send all sites not included in the Enterprise Mode Site List to Microsoft Edge." Enabling this setting automatically opens all sites that are not included in the Enterprise Mode Site List in Microsoft Edge. We recommend setting Microsoft Edge as the default browser when enabling this policy.