To enable IE Mode, you first need to enable the Flag "Enable IE Integration". Then when you open the page, open the "More" tool and select "Show this page using Internet Explorer" to change access.
It's worth noting that this Flag was launched a few weeks ago, but when it was enabled, IE mode would only be used as a stand-alone application. Now a new option appears in the drop-down menu. Users can switch between Default, Disabled, IE Mode and NeedIE according to their needs.
IE Mode has been supported in the latest Edge Dev release, but Edge for macOS platform does not support IE Mode. After all, IE browser is not supported on Mac devices, so there is no need for this.