A number of users who own second-generation Apple TV devices report noticing that the YouTube app, and any references to it in Settings, has vanished. The app remains on third-generation Apple TVs.
Late last month, Google updated the YouTube Data API to enable newer features and announced that older devices, including the second-generation Apple TV, would no longer be supported. This had the consequence of breaking the existing YouTube apps on a number of devices, including an assortment of HDTVs, Blu-ray players, and game consoles as well as iPhones, iPads, and iPod touch units running iOS 6 or earlier.
While the third-generation Apple TV was released in early 2012, many second-generation boxes remain in service. Apple seems to have moved the older boxes to end-of-life status, as only the third-generation models have recently been receiving new interface updates and apps.
It is widely believed that Apple will debut a new version of the Apple TV at its Worldwide Developers Conference in June. The model is rumored to sport an A8 processor, more internal storage, and App Store, and possibly Siri integration.