https://wccftech.com/youtube-is-testing-out-incognito-mode-for-its-android-app/

Incognito mode doesn’t really offer a whole lot when it comes to hiding your activity from three lettered agencies and is kind of pointless on a non-shared device. But, if you get the jitters everytime someone presses the P key on your keyboard, then it just might be the thing for you. Previously, only browsers had built in incognito mode functionality, but now, some Android apps offer an Incognito Mode as well. The only benefit to it is that can use their functionality without being tracked and without your actions being logged into your usage history. Gboard was one of the first apps to get it Today, it looks like YouTube is getting it too

YouTube always had an equivalent of incognito mode, where you had to navigate to Settings > History & privacy and toggling on the options to pause watch and search history. Another method was to sign out and use the app as is. But now it looks like YouTube will make a little easier to go into incognito without digging into many settings and without having to disable it later.