The last compatible version of YouTube for iOS 10.3.4 is YouTube v14.02.1 (sometimes v14.03, depending on the server configuration). This build was released in early 2019. It still supports the older UI, the old player, and crucially, the old authentication methods.
Sideloading means installing an IPA using your computer, without Apple’s App Store. This is the most common method. Youtube Ipa For Ios 10.3.4
: This only works if the app was previously linked to your Apple ID. 2. Version Spoofing (Requires Jailbreak) The last compatible version of YouTube for iOS 10