"You are not allowed to download any Content unless you see a “download” or similar link displayed by YouTube."
Old software lacks modern security patches, making it a target for malware or intrusion. Broken Functionality: youtube downloader old version 2.1 6 free download
You can keep the installer on a USB drive. For users in areas with slow or intermittent internet, re-installing version 2.1.6 is much faster than downloading a 150MB modern installer. "You are not allowed to download any Content