Modified IPAs from third-party sites can be risky as they aren't officially vetted by Apple.
: If you have previously downloaded YouTube on your Apple ID, go to App Store > Updates > Purchased youtube ipa for ios 12.5.6
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For the iOS 12.5.6 user, the official servers are closed. The only way to get YouTube is to obtain the IPA file from a third party. However, this is where the "walled garden" of Apple reveals its fortress-like nature. An IPA downloaded from the internet is not a self-contained executable. It is a locked box. To run it, it must be signed. Modified IPAs from third-party sites can be risky
Once installed, the vanilla YouTube IPA will still nag you to update. Here is how to disable that. it must be signed. Once installed