Bad news, it will discontinue support for older Apple devices. The game has been from the starting well supported and has been receiving regular updates with bug fixes and feature additions, like an improved AR mode.
To install the new build on your registered Apple TV, head over to Settings System Software Updates Update Software and hit the download button. This comes just a few days after beta 4 hit airwaves and a day after beta 5 of iOS 11.2.5 was released. A list of affected devices with pictures and model numbers can be found on the game's help page.
If your device isn't on that list than you'll be fine when the update drops but if it is then there isn't anything you can do but log into your Pokemon GO account on a different and compatible device to play.
After February 28, those using Apple devices not able to upgrade to iOS 11 (iPhone 5, iPhone 5C, iPad 4th Gen, iPad 3rd Gen, iPad mini, and iPad 2) will find themselves unable to access their Pokémon Go account. Hopefully, this new update to Pokemon GO is worth it.