If the VHD is removed after a Windows 11 upgrade, you can get it back or stop it from happening using the suggestions in this ...

How to Install Windows XP on Windows 8. Running two operating systems on one computer broadens your company's options when it comes to which software it can use.

Rename it with a VHD extension. In VirtualBox, open the VirtualXPVHD virtual machine, and voila! You've got Windows XP Mode (running Windows XP Professional) in Windows 8, no need for the XP license.