The contents of Windows.old folder can still be removed via Disk Cleanup, CCleaner and selecting the appropriate task or by running
After the August 2016 Anniversary update I was being given "Access Denied" in CMD:
RD /S /Q %SystemDrive%\Windows.old
The reason the folder itself is left behind is due to 2 files being locked by system files. Stopping the process WUDFHost.exe and processes/services associated with it will free up the process for an entire folder deletion. In my case there were oddly 2 instances of WUDFHost.exe running, stopping one instance fixed the Access Denied issue.
Sorry if this has already been mentioned, I got a headache reading.