If you install Windows 11 latest version from Windows insider and have the latest Edge canary, you can enable this flag, restart your browser and see the new changes in the Edge UI.
Edge menu on the right is transparent as well as right-click context menu on different parts of Edge, they are really nice, but the rest of the settings page is still in the old gray style. I know it's still work in progress, but I hope once Edge developers are done with it, they complete the transparent page for Edge and don't leave it like this.
the toolbar and title bar also should be transparent too.
hopefully Edge's settings page will eventually look like Windows 11's settings page.