It is possible to delete the Windows.old folder, but if you remove its contents, you will no longer be able to use the recovery options!
Doing a restore can be important sometimes, for example if you are not satisfied with the Windows 10/11 update, or encounter driver or compatibility issues, you can restore to the previous version of Windows 10/11 using the contents in Windows.old. If you delete the folder and then want to go back, you'll need to reinstall with the Windows version you want.
So deleting the Windows.old is only recommended if:
1. The space required on the hard drive
2. The new Windows 10 or 11 also works correctly after the update
3. All devices (printers, scanners, ...) work correctly
So it's part of the safety mechanism in case the update process fails. If the installation fails for some reason, the system can use the Windows.old folder to roll back the changes. When the upgrade completes successfully, the backup copy of the previous version is retained to provide the option to roll back in case the new installation causes problems. Although this is a handy feature, if your computer is working properly after the upgrade, you can safely delete the Windows.old folder to free up a few gigabytes of disk space.