This was left over from in place upgrade.
easiest way on servers is ccleaner
Correct way to delete it is using the “Disk Cleanup” utility from properties of C: drive in Windows Explorer.
In a server, the Disk Cleanup utility is not installed by default. You can get it by installing the Desktop Experience feature (under “User Interface” group). This will require a reboot.
As per this, the Windows.old folder may contain junctions (symlinks) so your results may vary!
You may notice no problems at first, but then when you reboot, and open Hyper-V Manager, you may see the message: “No virtual machines were found on this server” …
And you may find there are no files in %systemdrive%\programdata\Microsoft\Windows\Hyper-V\Virtual Machines\
You'll need to follow the advice of something like this - although note that I didn't need to follow all of the steps, so make sure you have good backups before starting, and use your common sense!
These are the only steps I needed to do (once per each VM):
Administrative Tools folder is missing from Control Panel