The desktop_ie11 segment is a compromise to meet IE halfway. Aa IE11 is still more used than it's replacement, Edge it could be a candidate for the desktop segment, but it is just not good enough. In some cases the lack of features are not important - in other cases they are vital. Giving it its own segment is the most flexible approach as developers can then individually decide how to group it with other segments.
As of December 2017 this segment represents ~6% of the desktop browsers and ~3% of all browsers (across all device types).
The desktop_ie11 segment is defined by the following criteria:
- Primary input method is mouse or trackpad
- Screen width of a minimum of 800 pixels
- Browser is Internet Explorer 11
Check out the complete feature support on caniuse.com.