Just played around with the "Allowsxs" option (Allow Microsoft Edge Side by Side browser experience). My experience on a 1809 OS with Edge stable version 80.0.361.66 is the following:
The shortcut "Microsoft Edge Legacy" does not always appear after the installation of Edge Stable. But if I start Edge with "microsoft-edge:" command, Edge legacy starts. So the Side-by-side works basically. But where is the shortcut?
Second, after uninstall of Edge Stable the shortcuts for Edge Legacy are broken. I just see a tile named "W~MSEdge" on my user profile.
On the release notes for Edge Stable 80.0.361.48 there was a fix for "Fixed an issue where Microsoft Edge and legacy Microsoft Edge side by side experience results in broken legacy links and crashes."
But as I said I use version 80.0.361.66 that still have similar issues.