Following through on an announcement that it made earlier this year, HBO has discontinued its app on 2nd and 3rd-generation Apple TVs, not just stopping updates for the app but actually removing it from the platform, meaning that users on those boxes won't be able to access it anymore.
A similar move is apparently coming for its other streaming service, HBO Go, ahead of the launch of new all-in-one platform HBO Max shortly. When HBO Now first launched it was an exclusive partner with Apple, although that arrangement ended after a while, potentially due to HBO realising that Apple's TV platform isn't the all-conquering behemoth that it can boast when it comes to smartphone sales.
The generations of Apple TV being abandoned by HBO are, in fairness, at least 8 years old in terms of when they were launched, but they're also relics from a time when Apple's set-top box was a more affordable device, and therefore had a decent level of popularity. Owners of those devices are a little up the river if they want to watch HBO on them now, and will likely have to look to alternative hardware.
Given that you could buy a third-generation box from Apple as recently as 2015, users do have grounds for disappointment on this front, even more so given that HBO Max will not be coming to the older platform when it does launch.
However, for now, third-generation Apple TV users can subscribe to HBO content through the main Apple TV app as the last remaining way to access HBO content on their device, which should be an acceptable middle ground, even if it might not last forever either.
Then again, with newer services launching all the time and similarly ignoring older platforms (as in the case of Disney+), these users might anyway have been looking to move on. Amazon's Fire TV sticks or one of Roku's lower-end devices would seem the most logical low-cost alternative at this stage.