updated 06:26 pm EDT, Wed June 18, 2014
Existing services to continue functioning for foreseeable future
As hinted at through the introduction of the iCloud-based Family Sharing feature, the latest beta of iOS for Apple TV (along with the corresponding Beta 2 of iOS 8) doesn't support second-generation Apple TV units, which were the first model to sport the small-black-box look of the current (third) generation devices. The second-generation models can be distinguished from the later version by not having support for 1080p output, and by having an Apple A4 SoC -- the same model that powers the iPhone 4, which is also no longer supported as of iOS 8.
It appears that the third-generation Apple TV will soon gain some of the new features touted for iOS 8, starting with the Family Sharing feature and apparently expanding to selected bits of "Continuity," a technology included in iOS 8 and OS X 10.10 Yosemite that allows seamless resumption of activities from device to device. On an Apple TV, it could be applied by offering the user the option of instantly continuing a song or video from the exact point it was stopped on a mobile device or Mac.
Because the four-year-old iPhone 4 is no longer supported in the forthcoming update to iOS, it makes sense that an Apple TV sporting similar hardware specs would also be deemed not compatible. While owners of the second-generation Apple TV (which originally debuted in September of 2010) will miss out on new features included in the iOS update for third-generation devices, the second-gen models are easily jailbroken, meaning that hacks such as ATV Flash (which enables additional applications and video codecs to be used on second-generation Apple TVs) can be employed, whereas the third-gen Apple TVs are not currently able to be jailbroken. Current Apple TVs rely on single-core A5 chips, the dual-core version of which powers the iPhone 4S, the iPad 2, the fifth-gen iPod touch and first-generation iPad mini.
Second-generation units won't be quickly abandoned, however. Assuming Apple maintains its usual tradition, second-gen owners can expect to continue to use the iTunes stores as well as third-party services like Hulu or Netflix for the foreseeable future. The older units might also continue to receive minor updates for security reasons or to add new channels, depending on feasibility.