The most common method for installing "tweaked" old YouTubes was via Enterprise Certificates (apps signed with a company profile). Companies like uYouPlus and iPAStore used to pay for these certificates.
Tweaks like and 3DAppVersionSpoofer can force the YouTube app to report a fake version number (e.g., telling Google’s server that v16 is actually v19). youtube old version ios patched
However, this is an arms race. Google now runs on the server side. Even if you spoof the version, the server notices that your client is missing modern HTTP headers and encryption protocols. After a few hours, the server deduces you are lying and bans the account or IP temporarily. The most common method for installing "tweaked" old