Apple will never officially support VP9, and why should they? It's rather stupid that Google uses it instead of H265/HEVC to begin with. But shouldn't Google be able to do a software implementation of VP9 decoder in their own YouTube-app for the Apple TV? There was no need with the previous Apple TV as it doesn't support 4K/HDR playback to begin with. But with the Apple TV 4K it becomes relevant and it's in Google/YouTube's best interest for their services to look at it's best.
Plex, Infuse and various other applications on the Apple TV are fully capable of doing local playback of files/formats and codecs that are not natively supported by the Apple TV. So there is obviously possible to do something in software to achieve support for codecs that are not directly supported by Apple?