You can add any custom folder under This PC in File Explorer in Windows 11/10 by editing the Registry by following this tutorial. In Windows 11/10 File Explorer Navigation Pane, if you want, you can ...
First, open the File Explorer using the Win+E hotkey and move to the target directory where you want to add comments to folders.
Windows File History is still the easiest, cheapest (free) continuous data protection software for Windows. The Add folder function has been removed, but it will back up Libraries you create yourself.