Even if you successfully installed a 2020 version of YouTube (v12.57) on a modern iPhone, it likely would not work.
When you download an app from the App Store, Apple serves the most recent version compatible with your device. You cannot select "Version 12.57" from a dropdown menu. The "download" link is dynamic and points to the current binary. download youtube ios 1257 link
Since there is no direct "link" to download version 12.57/1257 from Apple, users must use alternative, complex methods. Even if you successfully installed a 2020 version
Searching for a specific download link for an old app is a haven for cybercriminals. Here are the real dangers: When you download an app from the App
Even if you find a working download youtube ios 1257 link, the certificate used to install it will eventually be blacklisted by Apple. When that happens, the app crashes on open. You lose your downloads, playlists, and history.