We’ve reported Microsoft restricted Internet Explorer mode in Edge Chromium browser to enterprises only, the company has reconsidered their decision and now allows users to test IE mode for websites in new Edge browser, here is what you need to do to get it working again.

Rather than exposing option in the menu directly company now offers “Open Sites in Internet Explorer mode” option when you run Edge from the command line with a switch. With the next week’s update to Edge Dev, you’ll be informed on startup you’re using an unsupported command-line flag and warns you “this poses stability and security risks”, which you can ignore by clicking the close button.
These are the steps you need to follow to get Microsoft Edge’s IE mode back
Re-enable “Open sites in Internet Explorer mode option” in Chromium Edge
1. Ensure you’re using latest Dev version, which you can download and install it from here, visit edge://flags page
2. Search for “integration”, in the highlighted results, choose” IE mode for “Enable IE Integration” flag and restart the browser.
3. Exit Edge, right-click on its icon on the desktop and in the Target field after “C:Program Files (x86)MicrosoftEdge DevApplicationmsedge.exe” add the below the flag and click OK to apply the changes.
--ie-mode-test
The total string in the Target field should be like this for Dev Edge:
"C:Program Files (x86)MicrosoftEdge DevApplicationmsedge.exe" --ie-mode-test

4. Launch Edge browser by clicking on the shortcut to which we’ve added new command line flag, visit a website,
5. Click on Ellipsis menu, hover over “More Tools” to notice new “Open sites in Internet Explorer mode” option (formerly “show this page using Internet Explorer)

6. Select it, the page will open in IE mode and shows this message in info bar “All sites in this tab will be opened in Internet Explorer mode”, to test another site, just replace the current page web address or select option available in more Tools menu.
Note: The command line flag is working for dev build only, seems that’s intentional.
Update: the team checked in a cmdline flag that should unblock your scenario. Just add the flag “–ie-mode-test” to the launch parameters and the “Open in IE mode” menu will reappear.
Note: in Canary (and next week in Dev) you’ll get an reminder infobar on startup.
hth. pic.twitter.com/YPvQ7uczav
— Sean Lyndersay (@SeanOnTwt) July 29, 2019